regulator: max8998: Factor out struct voltage_map_desc

The struct voltage_map_desc is used for storing min/max/step fileds, and
then setup the min_uV, uV_step and n_voltages fields for regulator_desc.
Introduce MAX8998_LINEAR_REG/MAX8998_OTHERS_REG macro to setup these filds
directly in regulator_desc to simplify the code.
Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@kernel.org>