dm: led: Support toggling LEDs

Add support for toggling an LED into the uclass interface. This can be
efficiently implemented by the driver.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Ziping Chen <techping.chan@gmail.com>

include/led.h

@@ -20,6 +20,7 @@ struct led_uc_plat {
 enum led_state_t {
 	LEDST_OFF = 0,
 	LEDST_ON = 1,
+	LEDST_TOGGLE,
 
 	LEDST_COUNT,
 };