regulator: add buck3 in da903x driver

BUCK3 is the new component in DA9035. So there're three BUCKs in DA9035.
And there're two BUCKs in DA9034.

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>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>