atl1c: refine phy-register read/write function

phy register is read/write via MDIO control module ---
that module will be affected by the hibernate status,
to access phy regs in hib stutus, slow frequency clk must
be selected.
To access phy extension register, the MDIO related
registers are refined/updated, a _core function is
re-wroted for both regular PHY regs and extension regs.
existing PHY r/w function is revised based on the _core.
PHY extension registers will be used for the comming
patches.

Signed-off-by: xiong <xiong@qca.qualcomm.com>
Tested-by: Liu David <dwliu@qca.qualcomm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>