dt-bindings: regulator: Document bindings for fan53880

Add device tree binding information for fan53880 regulator driver.

Signed-off-by: Christoph Fritz <chf.fritz@googlemail.com>
Link: https://lore.kernel.org/r/20200702210846.31659-3-chf.fritz@googlemail.com


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