Bluetooth: btmtk: Add MT7928 support

Add support for MT7928 (internal device ID is MT7935) which requires
additional firmware (CBMCU firmware) loading before Bluetooth firmware.

CBMCU is a new component on MT7928 to handle common part shared across
the combo chip, providing a better user experience through improved
coordination between subsystems.

Implement two-phase CBMCU firmware download: Phase 1 loads section with
type 0x5 containing global descriptor, section maps and signature data;
Phase 2 loads remaining firmware sections. The firmware is rejected if it
has no Phase 1 section.

The CBMCU is shared with the Wi-Fi driver, which may download the same
firmware concurrently, so the firmware reports a status per section and
hands each one to a single driver. This driver downloads a section the
firmware reports as not downloaded, waits while the other driver holds
it, and leaves alone one the firmware already has. This is the scheme
btmtk_setup_firmware_79xx() already uses for the ROM patch shared with
Wi-Fi, so no host side lock is needed.

The driver then continues to load the corresponding BT firmware based on
device ID through fallthrough to case 0x7922/0x7925. A CBMCU failure is
only warned about and does not abort the setup, because the Wi-Fi driver
may finish the same download on its own. A CBMCU left unusable makes the
BT firmware download fail instead, which triggers the existing one-shot
btmtk_reset_sync() recovery.

The CBMCU firmware layout and section-map bounds are validated by
reusing btmtk_fw_validate_layout() and btmtk_fw_validate_section(),
the same helpers btmtk_setup_firmware_79xx() uses, instead of
duplicating the overflow checks.

Add the MT7928 (0x7935) case to btmtk_usb_subsys_reset(), which triggers
the subsystem reset by setting BIT(31) of MTK_BT_RESET_REG_CONNV3.

MT7928 bring-up kernel log:
[ 6931.197167] usb 1-3: New USB device found, idVendor=0e8d, idProduct=7935, bcdDevice= 1.00
[ 6931.197212] usb 1-3: New USB device strings: Mfr=5, Product=6, SerialNumber=7
[ 6931.197237] usb 1-3: Product: Wireless_Device
[ 6931.197258] usb 1-3: Manufacturer: MediaTek Inc.
[ 6931.197279] usb 1-3: SerialNumber: 000000000
[ 6931.213478] Bluetooth: hci1: Loading CBMCU firmware: mediatek/mt7928/CBMCU_CODE_MT7935_1_1.bin
[ 6931.215214] Bluetooth: hci1: CBMCU HW ver: 0x7935, SW ver: 0x0000, Build Time: 20260601T161751+
[ 6931.623962] Bluetooth: hci1: CBMCU firmware download completed
[ 6931.643916] Bluetooth: hci1: Loading BT firmware: mediatek/mt7928/BT_RAM_CODE_MT7935_1_1_hdr.bin
[ 6931.650467] Bluetooth: hci1: BT HW ver: 0x7935, SW ver: 0x0000, Build Time: 20260527000816
[ 6935.039790] Bluetooth: hci1: Device setup in 3369644 usecs
[ 6935.039833] Bluetooth: hci1: HCI Enhanced Setup Synchronous Connection command is advertised, but not supported.
[ 6935.160654] Bluetooth: hci1: AOSP extensions version v2.00
[ 6935.160710] Bluetooth: hci1: AOSP quality report is supported
[ 6935.162954] Bluetooth: MGMT ver 1.23

Signed-off-by: Chris Lu <chris.lu@mediatek.com>
Assisted-by: LLM
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>