ARM: dts: msm: IPA clock switches to virtual clock on msm8996 vplatform

Virtual clock driver has added support for RPM controlled clock.
So change the IPA clock which is controlled by RPM from dummy
clock to virtual clock.

Change-Id: If0a95fd0cb530c7a230105f5035373e7cac2c102
Signed-off-by: Zhiqiang Tu <ztu@codeaurora.org>