Fix warning

arch/arm/mach-omap2/pm.c: In function 'omap2_enter_full_retention':
arch/arm/mach-omap2/pm.c:504: warning: implicit declaration of
function 'omap2_gpio_prepare_for_retention'
arch/arm/mach-omap2/pm.c:531: warning: implicit declaration of
function 'omap2_gpio_resume_after_retention'

Signed-off-by: Dirk Behme <dirk.behme@gmail.com>