[POWERPC] Merge asm-ppc/pci-bridge.h into asm-power/pci-bridge.h

Moved bits need for ppc32 from asm-ppc/pci-bridge.h into
asm-powerpc/pci-bridge.h.

Removed ARCH=powerpc specific bits (and comments related to ARCH=ppc)
from asm-ppc/pci-bridge.h as its only used on ARCH=ppc.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>