nios2: add gpio_free

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Signed-off-by: Scott McNutt <smcnutt@psyent.com>
diff --git a/board/altera/nios2-generic/gpio.c b/board/altera/nios2-generic/gpio.c
index d449684..8c639ce 100644
--- a/board/altera/nios2-generic/gpio.c
+++ b/board/altera/nios2-generic/gpio.c
@@ -20,6 +20,11 @@
 	return 0;
 }
 
+int gpio_free(unsigned gpio)
+{
+	return 0;
+}
+
 int gpio_direction_input(unsigned gpio)
 {
 	u32 mask = 1 << gpio;