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>
diff --git a/doc/api/index.rst b/doc/api/index.rst
index ec0b8ad..9f7f23f 100644
--- a/doc/api/index.rst
+++ b/doc/api/index.rst
@@ -14,6 +14,7 @@
    event
    getopt
    interrupt
+   led
    linker_lists
    lmb
    logging