regulator: Add MAX8997_VOLTAGE_REGULATOR and MAX8997_CURRENT_REGULATOR macros

MAX8997_VOLTAGE_REGULATOR macro has _name and _ops parameters so we can use it
for all regulators with REGULATOR_VOLTAGE type.
The MAX8997_VOLTAGE_REGULATOR now replaces regulator_desc_ldo and
regulator_desc_buck macros.

MAX8997_CURRENT_REGULATOR is for CHARGER and CHARGER_TOPOFF.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>