[POWERPC] 85xx: Add default .config file for Wind River SBC8548

This is a suitable .config file for building the WRS SBC8548 kernel
to be used for NFS root via one of the TSEC interfaces and with
serial console via the soc/16550 compatible UART.
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>