mfd: rk808: Mark pm functions __maybe_unused

The newly added suspend/resume functions are only used if CONFIG_PM
is enabled:

drivers/mfd/rk808.c:752:12: error: 'rk8xx_resume' defined but not used [-Werror=unused-function]
drivers/mfd/rk808.c:732:12: error: 'rk8xx_suspend' defined but not used [-Werror=unused-function]

Mark them as __maybe_unused so the compiler can silently drop them
when they are not needed.

Fixes: 586c1b41 ("mfd: rk808: Add RK817 and RK809 support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>