MIPS: add asm/gpio.h to fix compilation error with CONFIG_CMD_GPIO.

With CONFIG_CMD_GPIO compilation reports error.
common/cmd_gpio.c:13:22: fatal error: asm/gpio.h: No such file or directory
 #include <asm/gpio.h>
                      ^
Signed-off-by: Purna Chandra Mandal <purna.mandal@microchip.com>
Reviewed-by: Chin Liang See <clsee@altera.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

arch/mips/include/asm/gpio.h

@@ -0,0 +1 @@
+#include <asm-generic/gpio.h>