clock-gcc-8994: Add 19.2MHz for the mock_utmi clock

The USB3 controller requires this clock to operate at
19.2MHz. Add this to the list of supported rates and
source it from the XO clock.

Change-Id: I0d3a4cc96aa1a203b2077cf8fa1215a0572f68b4
Signed-off-by: Jack Pham <jackp@codeaurora.org>