regulator: onsemi-ncp6335d: Add is_enabled API support

Without is_enabled function support, regualtor framework treats
it as an always on regulator and does not invoke the regulator enable
function call. Hence, add support for it.

Also, define a parent supply ("vin") for the regulator. This parent
can be used to pin-control the regulator. While at it, change the initcall
level to subsys_init. so that the driver registers after the parent supply
("vin" - provided by fixed-regualtor).

Change-Id: I93e870d2e4b89f7874c3e17be91bc417dadf1777
Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>