net/fsl_pq_mdio: Allow explicit speficition of TBIPA address

This introduces a simpler and generic method for for finding (and mapping)
the TBIPA register.

Instead of relying of complicated logic for finding the TBIPA register
address based on the MDIO or MII register block base
address, which even in some cases relies on undocumented shadow registers,
a second "reg" entry for the mdio bus devicetree node specifies the TBIPA
register.

Backwards compatibility is kept, as the existing logic is applied when
only a single "reg" mapping is specified.

Signed-off-by: Esben Haabendal <eha@deif.com>
Signed-off-by: David S. Miller <davem@davemloft.net>