dt-bindings: leds: Add binding for sgm3140

Add YAML devicetree binding for SGMICRO SGM3140 charge pump used for
camera flash LEDs.

Signed-off-by: Luca Weiss <luca@z3ntu.xyz>
Reviewed-by: Dan Murphy <dmurphy@ti.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Pavel Machek <pavel@ucw.cz>