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/drivers/Kconfig b/drivers/Kconfig
index c7e526c..5ebc89d 100644
--- a/drivers/Kconfig
+++ b/drivers/Kconfig
@@ -20,6 +20,8 @@
 
 source "drivers/input/Kconfig"
 
+source "drivers/led/Kconfig"
+
 source "drivers/serial/Kconfig"
 
 source "drivers/tpm/Kconfig"