powerpc/85xx: Display SERDES 8 erratum warning if banks are not disabled

The work-around for P4080 erratum SERDES-8 requires all lanes of banks two
and three to be disabled (powered down) in the RCW.  Display a warning
message if this is not the case.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>