net: phy: mediatek: add the missing suspend/resume callbacks

Without suspend/resume callbacks, the PHY cannot be powered down/up
administratively.

Fixes: e40d2cca

 ("net: phy: add MediaTek Gigabit Ethernet PHY driver")
Signed-off-by: DENG Qingfang <dqfext@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20210823044422.164184-1-dqfext@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>