ARM: 6280/1: imx: Fix build failure when including <mach/gpio.h> without <linux/spinlock.h>

This is a follow up to

	14cb0deb

 (arm/imx/gpio: add spinlock protection)

and fixes the following build failure:

	  CC      arch/arm/mach-imx/pcm970-baseboard.o
	In file included from arch/arm/include/asm/gpio.h:6,
			 from include/linux/gpio.h:8,
			 from arch/arm/mach-imx/pcm970-baseboard.c:20:
	arch/arm/plat-mxc/include/mach/gpio.h:40: error: expected specifier-qualifier-list before 'spinlock_t'

Cc: stable@kernel.org
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>