USB: EHCI: Add support for overriding corner voltage levels for VDDCX

Currently ehci-msm2 driver does have support for voting corner voltages
for VDDCX. These corner voltage levels for VDDCX will change from platform
to platform. Hence add support for passing these corner voltage levels for
VDDCX from device tree file.

Change-Id: Id5d8c1d435721d6e2d1640298685b3f701fc3d6e
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>