regulator: s2mpxxx: Move regulator min/step voltages in common place

This is a cleanup patch and moves min/step voltages in a common samsung
header file so that they can be used by other s2mpxxx PMIC drivers. Only
few required macros are added currently and others can be added if needed.

Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Amit Daniel Kachhap <amit.daniel@samsung.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>