regulator: max8997: Avoid spaces in regulator names

max8997-pmic instantiated from device tree uses names, not numerical
ids to distinguish between outputs.

Replace spaces with underscores in said names to make it possible to
describe these outputs as regulators in DTS.

Signed-off-by: Karol Lewandowski <k.lewandowsk@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>