regulator: onsemi-ncp6335d: add support for device tree

Add device tree support. Add support for restoring working register
from back up register. Add support for setting vsel pin by gpio output
according to device tree flag. Change ncp6335d_regulator_init into a
global function.

Signed-off-by: Ke Liu <keliu@codeaurora.org>
Change-Id: I5ce861dd307b28e0782d051b105956b10eb1afab