leds: lgm: Add LED controller driver for LGM SoC

Parallel to serial conversion, which is also called SSO controller,
can drive external shift register for LED outputs, reset or
general purpose outputs.

This driver enables LED support for Serial Shift Output Controller (SSO).

Signed-off-by: Amireddy Mallikarjuna reddy <mallikarjunax.reddy@linux.intel.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>