powerpc/85xx: implement check for erratum A-004580 work-around

The work-around for erratum A-004580 ("Internal tracking loop can falsely
lock causing unrecoverable bit errors") is implemented via the PBI
(pre-boot initialization code, typically attached to the RCW binary).
This is because the work-around is easier to implement in PBI than in
U-Boot itself.

It is still useful, however, for the 'errata' command to tell us whether
the work-around has been applied.  For A-004580, we can do this by verifying
that the values in the specific registers that the work-around says to
update.

This change requires access to the SerDes lane sub-structure in
serdes_corenet_t, so we make it a named struct.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>