regulator: max77802: Add header for operating modes

Add a header file for the max77802 constants that could be shared between
the regulator driver and Device Tree source files. Also, remove standby
and off opmodes since only normal and low power are valid operating modes.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Mark Brown <broonie@kernel.org>