[POWERPC] 85xx: Add basic support for Wind River SBC8548 board

This adds the basic support for the Wind River SBC8548 board, implemented
as powerpc.  It closely follows the implementation of the MPC8548CDS.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>