regulator: onsemi-ncp6335d: add device tree support for mode selection

Add support to the onsemi-ncp6335d driver to select which regulator
operating mode to utilize. There are two possible mode selections: PWM mode
and auto mode. If no mode property is specified in device tree, the
regulator will operate in the hardware default mode.

CRs-Fixed: 607899
Change-Id: I2e21c452a3ea3963146d9abbc7e89e811fd90388
Signed-off-by: Ke Liu <keliu@codeaurora.org>