phy: qcom-sgmii-eth: add .set_mode() and .validate() methods

qcom-sgmii-eth is an Ethernet SerDes supporting only Ethernet mode
using SGMII, 1000BASE-X and 2500BASE-X.

Add an implementation of the .set_mode() method, which can be used
instead of or as well as the .set_speed() method. The Ethernet
interface modes mentioned above all have a fixed data rate, so
setting the mode is sufficient to fully specify the operating
parameters.

Add an implementation of the .validate() method, which will be
necessary to allow discovery of the SerDes capabilities for platform
independent SerDes support in the stmmac network driver.

Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>
Acked-by: Vinod Koul <vkoul@kernel.org>
Tested-by: Mohd Ayaan Anwar <mohd.anwar@oss.qualcomm.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1vvkU3-0000000AuP2-0hu3@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>