Input: gpio_keys - use <linux/gpio.h> instead of <asm/gpio.h>

The gpio keys driver should be using <linux/gpio.h> instead
of <asm/gpio.h>

Signed-off-by: Ben Dooks <ben@simtec.co.uk>
Signed-off-by: Simtec Linux Team <linux@simtec.co.uk>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>