net: ethernet: mediatek: add controlling power domain the ethernet belongs to

introduce power domain control which the digital circuit of
the ethernet belongs to inside the flow of hardware initialization
and deinitialization which helps the entire ethernet hardware block
could restart cleanly and completely as being back to the initial
state when the whole machine reboot.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>