mfd: ipaq-micro: Use __maybe_unused to hide pm functions

The ipaq-micro driver uses SET_SYSTEM_SLEEP_PM_OPS() to
remove the reference to its resume function, but does
not use an #ifdef around the definition, so we get
a build warning:

drivers/mfd/ipaq-micro.c:379:12: error: 'micro_resume' defined but not used [-Werror=unused-function]

This adds a __maybe_unused annotation so the compiler knows
it can silently drop it instead of warning.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>