[MIPS] Create fallback gpio.h

Create fallback gpio.h which only contains prototypes for gpio API.
Signed-off-by: NAtsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: NRalf Baechle <ralf@linux-mips.org>

include/asm-mips/mach-generic/gpio.h

+#ifndef __ASM_MACH_GENERIC_GPIO_H
+#define __ASM_MACH_GENERIC_GPIO_H
+
+int gpio_request(unsigned gpio, const char *label);
+void gpio_free(unsigned gpio);
+int gpio_direction_input(unsigned gpio);
+int gpio_direction_output(unsigned gpio, int value);
+int gpio_get_value(unsigned gpio);
+void gpio_set_value(unsigned gpio, int value);
+int gpio_to_irq(unsigned gpio);
+int irq_to_gpio(unsigned irq);
+
+#include <asm-generic/gpio.h>		/* cansleep wrappers */
+
+#endif /* __ASM_MACH_GENERIC_GPIO_H */