irqchip: gic-v3: Tag all low level accessors __maybe_unused

This is only really needed for gic_write_sgi1r in the !SMP case since it
is only referenced in the SMP initialisation code but it seems better to
have these functions all next to each other and declared consistently.

Signed-off-by: Mark Brown <broonie@linaro.org>
Link: https://lkml.kernel.org/r/1406748194-21094-1-git-send-email-broonie@kernel.org
Signed-off-by: Jason Cooper <jason@lakedaemon.net>