phy-msm-ssusb-qmp: Toggle between xo and pipe clk in/out of LPM

As per HPG, driver needs to toggle between bi_tcxo and phy pipe
clock as part of its CX collapse sequence. This is done by
setting pipe_clk/ref_clk_src as parent of pipe_clk_src.

Change-Id: I353cc55724c8a2bf6b38d4a7f9a1e6f573bf3cab
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>