ARM: p2v: factor out BE8 handling

The big and little endian versions of the ARM p2v patching routine only
differ in the values of the constants, so factor those out into macros
so that we only have one version of the logic sequence to maintain.

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