add generic stubs for GPIO LEDs

For boards that support LEDs driven via GPIO (CONFIG_GPIO_LED),
it may be useful to have some generic stubs (wrapper functions)
for the "colored" LEDs.

This allows defining STATUS_LED_* values directly to GPIO numbers,
e.g.: #define STATUS_LED_GREEN	248 /* = PH24 */

To keep those optional, it's probably best to introduce an additional
configuration setting. I've chosen CONFIG_GPIO_LED_STUBS for that.
Placing the code in drivers/misc/gpio_led.c also ensures that it
automatically depends on CONFIG_GPIO_LED too.

Signed-off-by: Bernhard Nortmann <bernhard.nortmann@web.de>
Reviewed-by: Simon Glass <sjg@chromium.org>