backlight: pm8941-wled: Add default-brightness property

Default the brightness to 2048 and add possibility to override this in
device tree.

Suggested-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>