dt-bindings: mms114: document melfas,mms345l binding

The mms114 driver now supports MMS345L; document the
melfas,mms345l binding that is used for it.
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Reviewed-by: Andi Shyti <andi@etezian.org>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200423102431.2715-2-stephan@gerhold.netSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

Documentation/devicetree/bindings/input/touchscreen/mms114.txt

-* MELFAS MMS114/MMS152 touchscreen controller
+* MELFAS MMS114/MMS152/MMS345L touchscreen controller
 
 Required properties:
 - compatible: should be one of:
 	- "melfas,mms114"
 	- "melfas,mms152"
+	- "melfas,mms345l"
 - reg: I2C address of the chip
 - interrupts: interrupt to which the chip is connected
 - touchscreen-size-x: See [1]