dt-bindings: mfd: LMU: Add ti,brightness-resolution

Add ti,brightness-resolution to the TI LMU binding to define
whether the device uses 8-bit brightness or 11-bit brightness.

Signed-off-by: Dan Murphy <dmurphy@ti.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>