common: fix include guards for CONFIG_MP

This was breaking the build for some boards:
MPC8536DS MPC8536DS_36BIT MPC8536DS_SDCARD MPC8536DS_SPIFLASH qemu-ppce500

Include only these features for some PPC boards if the configuration for MultiProcessor
is enabled.

Signed-off-by: Gabriel Huau <contact@huau-gabriel.fr>
Cc: Tom Rini <trini@ti.com>
Cc: York Sun <yorksun@freescale.com>
Acked-by: York Sun <yorksun@freescale.com>