WATCHDOG: fix build PM warnings

drivers/watchdog/sp805_wdt.c:288:12: warning: 'sp805_wdt_suspend' defined but not used
drivers/watchdog/sp805_wdt.c:298:12: warning: 'sp805_wdt_resume' defined but not used

This is caused by the wrong config symbol being used for these functions.
Rather than fixing that, mark the functions with __maybe_unused

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>