regulator: max77686: Replace hard-coded opmode values with defines

Add defines for regulator operating modes which should be more readable,
especially if one does not have Maxim 77686 datasheet.

The patch does not introduce any functional change.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Suggested-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Mark Brown <broonie@kernel.org>