[ARM] Allow r2 to be passed through the decompressor to the kernel

This is part of a patch from Marc Singer to allow r2 to be
passed to the kernel.  Marc's original comments follow:

This revised R2 (atags pointer) patch incorporates comments from Nico
Pitre and Ben Dooks. It modifies the head.S files such that the R2
value set by the bootloader is conveyed to the kernel startup code.
The kernel head.S heuristically validates the pointer. It will set R2
to zero if it believes the pointer is invalid. Presently, it requires
that the ATAGS list reside in the first 16KiB of physical RAM.
Relaxing this contraint may be both desirable as well as tricky.

Signed-off-by: Marc Singer <elf@buici.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>