The operand is an integer constant, make the constness explicit by
adding the modifier. This is needed for clang to generate valid code
and also works with gcc.

Also change the constraint of the operand from 'I' ("Integer constant
that is valid as an immediate operand in an ADD instruction", AArch64)
to 'i' ("An immediate integer operand").

Based-on-patch-from: Greg Hackmann <ghackmann@google.com>
Signed-off-by: Greg Hackmann <ghackmann@google.com>
Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>