dt-bindings: mfd: Add TI LMU device binding information

This patch describes overall binding for TI LMU MFD devices.

Signed-off-by: Milo Kim <milo.kim@ti.com>
Acked-by: Rob Herring <robh+dt@kernel.org>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>