leds: tps6105x: add driver for MFD chip LED mode

This driver adds support for the LED operational mode of the
tps6105x MFD device.

Acked-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Signed-off-by: Sven Van Asbroeck <TheSven73@gmail.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>