regulator: dbx500-prcmu: Remove operating_point from struct dbx500_regulator_info

operating_point is not used since the initial commit of db8500-prcmu driver,
remove it.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Bengt Jonsson <bengt.g.jonsson@stericsson.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>