usb: ehci-msm2: Add support for separate PHY

Newer MSM targets may have ChipIdea USB host controller paired
with different PHYs, which will have a separate driver to handle
initialization and reset sequences. In this case we do not need
to access the USB_PHY_CTRL register or the vendor-specific
extended ULPI_VIEWPORT registers.

Add support for getting the PHY instance from a devicetree phandle,
and use the usb_phy API to interface with it. This will make
way for future decoupling of the PHY from the controller.

Change-Id: I40dea227fc4a220829362e847ff4f580d1fce6ad
Signed-off-by: Jack Pham <jackp@codeaurora.org>