kernel: backport MediaTek Ethernet PHY driver

kernel: backport MediaTek Ethernet PHY driver

Add support for MediaTek Gigabit Ethernet PHYs found in MT7530 and
MT7531. Fix some link up/down issues.
The errornous check for the PHY mode which broke things with MT7531
has been removed as suggested by patch
 net: phy: mediatek: remove PHY mode check on MT7531
As a result, things are working fine now on MT7622+MT7531 as well.

Signed-off-by: DENG Qingfang <dqfext@gmail.com>
Tested-by: Daniel Golle <daniel@makrotopia.org>
Tested-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Tested-by: Stijn Tintel <stijn@linux-ipv6.be>