drivers: regulator: Add support for OnSemi NCP6335D regulator

This driver supports OnSemi NCP6335D synchronous buck converter.
The NCP6335D is a step-down switching voltage regulator which
provides a voltage range from 0.6 to 1.4V. The regulator is
controlled via the I2C interface.

Change-Id: I237fe781d5c54d2b3ffc65096b9e83c387d3b6f5
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>