HID: asus: add support for xgm led

XG mobile stations have very bright leds behind the fan that can be
turned either ON or OFF: add a cled interface to allow controlling the
brightness of those red leds.

Let the led core manage the power transitions: the classdev is flagged
with LED_CORE_SUSPENDRESUME, so it is switched off at suspend and its
last brightness is restored at resume. The EC drives its own blinking
pattern during s2idle anyway, so the led state while the machine is
asleep is not meaningful.

Signed-off-by: Denis Benato <denis.benato@linux.dev>
Signed-off-by: Jiri Kosina <jkosina@suse.com>