[ARM] omap: fix gpio.c build error

arch/arm/plat-omap/gpio.c: In function '_omap_gpio_init':
arch/arm/plat-omap/gpio.c:1492: error: 'omap_mpuio_device' undeclared (first use in this function)

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