dm: led: Tidy up SPL options for the led and led-gpio

At present SPL does not have its own option. But these features can
increase SPL code size. Adjust the Kconfig and Makefile so that
separate a SPL option can be selected.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/led/Makefile b/drivers/led/Makefile
index 990129e..02367fd 100644
--- a/drivers/led/Makefile
+++ b/drivers/led/Makefile
@@ -5,5 +5,5 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-obj-$(CONFIG_LED) += led-uclass.o
-obj-$(CONFIG_LED_GPIO) += led_gpio.o
+obj-y += led-uclass.o
+obj-$(CONFIG_$(SPL_)LED_GPIO) += led_gpio.o