regulator: max77686: Make regulator_desc array const

The regulator_register() expects array of 'regulator_desc' to be const.

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