regulator: bindings: add document bindings for mpq7920

Add device tree binding information for mpq7920 regulator driver.
Example bindings for mpq7920 are added.

Signed-off-by: Saravanan Sekar <sravanhome@gmail.com>
Link: https://lore.kernel.org/r/20200108131234.24128-3-sravanhome@gmail.com


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