asm-generic: xor: mark static functions as __maybe_unused

The asm-generic/xor.h header file is nasty and defines static functions
that are not inline. The header file is include by the ARM version of
asm/xor.h, which uses some but not all of the symbols defined there.

Marking the extraneous functions as __maybe_unused lets gcc drop them
without complaining.

Without this patch, building iop13xx_defconfig results in:

include/asm-generic/xor.h:696:34: warning: 'xor_block_8regs_p' defined but not used [-Wunused-variable]
include/asm-generic/xor.h:704:34: warning: 'xor_block_32regs_p' defined but not used [-Wunused-variable]

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Dan Williams <dan.j.williams@gmail.com>
Cc: Neil Brown <neilb@suse.de>