arm/dts: 8974: Use corner voltages for VDDCX voting for ehci-msm2 driver

Currently ehci-msm2 driver is voting for discrete voltages for VDDCX
instead of corner voltages. This could cause issue as RPM may not able
to handle this descrete voltage request for VDDCX on some platforms.
Hence vote for corner voltage for VDDCX instead of discrete voltages
on new platforms.

Change-Id: Iacdaca099252832e37a6653781b03528808e1ebd
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>