dt-bindings: leds: Add bindings for Intel LGM SoC

Add DT bindings YAML schema for SSO controller driver
of Lightning Mountain (LGM) SoC.

Signed-off-by: Amireddy Mallikarjuna reddy <mallikarjunax.reddy@linux.intel.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>
Reviewed-by: Rob Herring <robh@kernel.org>