net: phy: icplus: split IP101A/G driver

Unfortunately, the IP101A and IP101G share the same PHY identifier.
While most of the functions are somewhat backwards compatible, there is
for example the APS_EN bit on the IP101A but on the IP101G this bit
reserved. Also, the IP101G has many more functionalities.

Deduce the model by accessing the page select register which - according
to the datasheet - is not available on the IP101A. If this register is
writable, assume we have an IP101G.

Split the combined IP101A/G driver into two separate drivers.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>