hwrng: omap3-rom - Fix unused function warnings

When runtime-pm is disabled, we get a few harmless warnings:

drivers/char/hw_random/omap3-rom-rng.c:65:12: error: unused function 'omap_rom_rng_runtime_suspend' [-Werror,-Wunused-function]
drivers/char/hw_random/omap3-rom-rng.c:81:12: error: unused function 'omap_rom_rng_runtime_resume' [-Werror,-Wunused-function]

Mark these functions as __maybe_unused so gcc can drop them
silently.

Fixes: 8d9d4bdc

 ("hwrng: omap3-rom - Use runtime PM instead of custom functions")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>