gpio: support gpio set/get pull status

On some platform gpio can set/get pull status when input, add these
function so we can set/get gpio pull status when need it. And they are
optional function.
diff --git a/drivers/gpio/gpio.c b/drivers/gpio/gpio.c
index c06172f..ef6bb9c 100644
--- a/drivers/gpio/gpio.c
+++ b/drivers/gpio/gpio.c
@@ -80,6 +80,26 @@
 	ops->set_value(gpio, value);
 }
 
+void gpio_set_pull(int gpio, int pull)
+{
+	assert(ops);
+	assert(ops->set_pull != 0);
+	assert((pull == GPIO_PULL_NONE) || (pull == GPIO_PULL_UP) ||
+	       (pull == GPIO_PULL_DOWN));
+	assert(gpio >= 0);
+
+	ops->set_pull(gpio, pull);
+}
+
+int gpio_get_pull(int gpio)
+{
+	assert(ops);
+	assert(ops->get_pull != 0);
+	assert(gpio >= 0);
+
+	return ops->get_pull(gpio);
+}
+
 /*
  * Initialize the gpio. The fields in the provided gpio
  * ops pointer must be valid.
diff --git a/include/drivers/gpio.h b/include/drivers/gpio.h
index a5cb5c7..633b3f6 100644
--- a/include/drivers/gpio.h
+++ b/include/drivers/gpio.h
@@ -37,17 +37,25 @@
 #define GPIO_LEVEL_LOW		0
 #define GPIO_LEVEL_HIGH		1
 
+#define GPIO_PULL_NONE		0
+#define GPIO_PULL_UP		1
+#define GPIO_PULL_DOWN		2
+
 typedef struct gpio_ops {
 	int (*get_direction)(int gpio);
 	void (*set_direction)(int gpio, int direction);
 	int (*get_value)(int gpio);
 	void (*set_value)(int gpio, int value);
+	void (*set_pull)(int gpio, int pull);
+	int (*get_pull)(int gpio);
 } gpio_ops_t;
 
 int gpio_get_direction(int gpio);
 void gpio_set_direction(int gpio, int direction);
 int gpio_get_value(int gpio);
 void gpio_set_value(int gpio, int value);
+void gpio_set_pull(int gpio, int pull);
+int gpio_get_pull(int gpio);
 void gpio_init(const gpio_ops_t *ops);
 
 #endif	/* __GPIO_H__ */