leds: add sgm3140 driver

Add a driver for the SGMICRO SGM3140 Buck/Boost Charge Pump LED driver.

This device is controlled by two GPIO pins, one for enabling and the
second one for switching between torch and flash mode.

Signed-off-by: Luca Weiss <luca@z3ntu.xyz>
Signed-off-by: Pavel Machek <pavel@ucw.cz>