regulator: Add MAX8893 bindings

Add Maxim MAX8893 PMIC device tree bindings. The example is also
provided.

Signed-off-by: Sergey Larin <cerg2010cerg2010@mail.ru>
Link: https://lore.kernel.org/r/20210618141607.884-2-cerg2010cerg2010@mail.ru


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