Patch from Ian Campbell

Arecent changeset[0] that changed params into an asm function call
broke the PXA build since PXA does not define PARAMS_PHYS therefore
the params_phys variable is never defined and linking arch/arm/boot/compressed/head.S fails.

The function is defined in include/asm-arm/arch-rpc/uncompress.h 
and so I assume it is dependendant on CONFIG_ARCH_RPC. I can't find 
any use of it outside this file.

Signed-off-by: Ian Campbell