doc: introduce led.rst documentation

Introduce simple led.rst documentation to document all the additional
Kconfig and the current limitation of LED_BLINK and GPIO software blink.

Also add missing definition for sw_blink in led_uc_plat struct.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
3 files changed