msm: clock-9615: Prevent usb_hsic_sys_clk from voting on CXO

Due to a hardware limitation, it may be required that
the USB HSIC System clock remain on during suspend and
XO shutdown. Use the pll8_acpu_clk as the PLL source for
the clock.

Change-Id: Id92f6139d867534ee4486395903b6b1482c9f7d9
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>