Documentation: mfd: Add DT bindings for the LP873X

The lp873x series of PMICs have a bunch of regulators and a couple
of GPO(General Purpose Outputs).
Add information for the MFD and regulator drivers.

Signed-off-by: Keerthy <j-keerthy@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>