regulator: Document bindings for max77826

Add device tree binding information for max77826 regulator driver.

Signed-off-by: Iskren Chernev <iskren.chernev@gmail.com>
Link: https://lore.kernel.org/r/20200414172250.2363235-3-iskren.chernev@gmail.com


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