spi: rpc-if: Remove CONFIG_PM_SLEEP ifdefery

Use __maybe_unused for the suspend()/resume() hooks and get rid of
the CONFIG_PM_SLEEP ifdefery to improve the code.

Suggested-by: Pavel Machek <pavel@denx.de>
Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Acked-by: Pavel Machek <pavel@denx.de>
Link: https://lore.kernel.org/r/20201230145708.28544-3-prabhakar.mahadev-lad.rj@bp.renesas.com


Signed-off-by: Mark Brown <broonie@kernel.org>