regulator: support da9030 BUCK in da903x driver

Support the operation of DA9030 BUCK2 in da903x driver.

Signed-off-by: Haojian Zhuang <haojian.zhuang@marvell.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Eric Miao <eric.y.miao@gmail.com>
Acked-by: Mike Rapoport <mike@compulab.co.il>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>