USB: OTG: Move vbus power routine from board file to otg driver

Regulator framework provides pseudo regulator called vbus_otg which
encapsulates the vbus power-up sequence. "vbus_otg" is software
name used for the vbus power-up sequence. Based on the platform,
vbus power-up sequence related interfaces ( gpios, MPPs, LDOs, vbus
switch ) would get enable with enabling this pseudo regulator.

With this change, remove vbus power-up routine from board file.
Use "vbus_otg" pseudo regulator to enable/disable the vbus power-up
sequence from otg driver.

Change-Id: I416aa03a7b743f8a8f6dffe591342688e3bc3de6
Signed-off-by: Mayank Rana <mrana@codeaurora.org>