net: pcs: xpcs: add support for NXP SJA1110

The NXP SJA1110 switch integrates its own, non-Synopsys PMA, but it
manages it through the register space of the XPCS itself, in a small
register window inside MDIO_MMD_VEND2 from address 0x8030 to 0x806e.

This coincides with where the registers for the default Synopsys PMA
are, but the register definitions are of course not the same.

This situation is an odd hardware quirk, but the simplest way to manage
it is to drive the SJA1110's PMA from within the XPCS driver.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>