ARM: dts: msm: switch to virtual clock for usb3 on msm8996 vplatform

Virtual clock driver is ready and gvm needs to manage clocks of usb3
which is a typical pass through device.

Change-Id: Ie8edfce2602f090d0f6b59d0af2e4b39282dfd6d
Signed-off-by: Yimin Peng <yiminp@codeaurora.org>