ARM: p2v: move patching code to separate assembler source file

Move the phys2virt patching code into a separate .S file before doing
some work on it.

Suggested-by: Nicolas Pitre <nico@fluxnic.net>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>