nios2: add gpio_free

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Signed-off-by: Scott McNutt <smcnutt@psyent.com>

board/altera/nios2-generic/gpio.c

@@ -20,6 +20,11 @@ int gpio_request(unsigned gpio, const char *label)
 	return 0;
 }
 
+int gpio_free(unsigned gpio)
+{
+	return 0;
+}
+
 int gpio_direction_input(unsigned gpio)
 {
 	u32 mask = 1 << gpio;