ARM: dts: msm: Add ref_clk_src to QUSB PHYs on msm8996

The QUSB PHY instances each require a ref clk sourced by PMIC
ln_bb_clk in order to function properly. Since this clock is
shared among other peripherals, make sure the PHYs also can
enable it independently when needed.

Change-Id: Id5837532a2c9249b7babb720483c94734d80b717
Signed-off-by: Jack Pham <jackp@codeaurora.org>