dm: led: Add support for getting the state of an LED

It is useful to be able to read the LED as well as write it. Add this to
the uclass and update the GPIO driver.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Ziping Chen <techping.chan@gmail.com>
diff --git a/test/dm/led.c b/test/dm/led.c
index ebb9b46..68aa39b 100644
--- a/test/dm/led.c
+++ b/test/dm/led.c
@@ -43,9 +43,11 @@
 	ut_asserteq(0, sandbox_gpio_get_value(gpio, offset));
 	ut_assertok(led_set_state(dev, LEDST_ON));
 	ut_asserteq(1, sandbox_gpio_get_value(gpio, offset));
+	ut_asserteq(LEDST_ON, led_get_state(dev));
 
 	ut_assertok(led_set_state(dev, LEDST_OFF));
 	ut_asserteq(0, sandbox_gpio_get_value(gpio, offset));
+	ut_asserteq(LEDST_OFF, led_get_state(dev));
 
 	return 0;
 }