powerpc/p2041: fix serdes reference clock frequency display for PC board

powerpc/p2041: fix serdes reference clock frequency display for PC board

PC board has different serdes clock setting with PB board, it uses same
serdes frequency setting on bank2 as on bank1. PC board can be distingushed
from PB board by checking CPLD version, if running on PC board, then fix
the serdes reference clock frequency of bank2.

Signed-off-by: Shaohui Xie <Shaohui.Xie@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>