USB: dwc3-msm: Simplify design for VDDCX voting

Discrete voltages and corner values for voting VDDCX might
change from platform to platform. Hence add support for
passing either voltage values or corner values from device tree.

Change-Id: Ib96a1da618a94123ed3aea9f0c6be9df43e1fd4e
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>