net: phy: add MediaTek Gigabit Ethernet PHY driver

Add support for MediaTek Gigabit Ethernet PHYs found in MT7530 and
MT7531 switches.
The initialization procedure is from the vendor driver, but due to lack
of documentation, the function of some register values remains unknown.

Signed-off-by: DENG Qingfang <dqfext@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>