usb: phy: msm: Expose DP/DM operations as a regulator

Instead of having the controller driver call the PHY's DP/DM
properties on behalf of the charger driver to enable and disable
the regulator, allow the PHY to expose a regulator device.
The charger driver then obtains a handle to this regulator to
perform regulator enable and disable directly.

Change-Id: I24c9b5722d84ea90f5a339245eb85b6a0dcbccf0
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>