Documentation: Add LP3943 DT bindings and document

Bindings for LP3943 MFD, GPIO and PWM controller are added.

Signed-off-by: Milo Kim <milo.kim@ti.com>
Acked-by: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>