dm: Add support for LEDs

Add a simple uclass for LEDs, so that these can be controlled by the device
tree and activated when needed. LEDs are referred to by their label.

This implementation requires a driver for each type of LED (e.g GPIO, I2C).

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/dm/uclass-id.h b/include/dm/uclass-id.h
index 24baa5c..a961648 100644
--- a/include/dm/uclass-id.h
+++ b/include/dm/uclass-id.h
@@ -33,6 +33,7 @@
 	UCLASS_I2C,		/* I2C bus */
 	UCLASS_I2C_EEPROM,	/* I2C EEPROM device */
 	UCLASS_I2C_GENERIC,	/* Generic I2C device */
+	UCLASS_LED,		/* Light-emitting diode (LED) */
 	UCLASS_LPC,		/* x86 'low pin count' interface */
 	UCLASS_MASS_STORAGE,	/* Mass storage device */
 	UCLASS_MOD_EXP,		/* RSA Mod Exp device */