dt-bindings: regulator: add document bindings for mp5416

Add device tree binding information for mp5416 regulator driver.

Signed-off-by: Saravanan Sekar <sravanhome@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200204110419.25933-2-sravanhome@gmail.com


Signed-off-by: Mark Brown <broonie@kernel.org>