usb: phy: qmp: Add support to use different voltage with core supply

On newer platform USB QMP PHY needs different voltage supply as core
voltage. This change adds required support for the same.
Also Rename vdda18-supply as core-supply for USB QMP PHY for targets
based on 3.18 kernel.

CRs-Fixed: 1001463
Change-Id: If100d36bade241dedf28e3cea9e07be192bdfdc2
Signed-off-by: Mayank Rana <mrana@codeaurora.org>