msm: pcie: add support to control pipe clk mux for LPM

PCIe driver needs to toggle between bi_tcxo and phy pipe
clock as part of its LPM sequence. This is done by setting
pipe_clk/ref_clk_src as parent of pipe_clk_src.

Change-Id: Ie634e6024fef3e5736793fc53d281be2d52bcbf7
Signed-off-by: Tony Truong <truong@codeaurora.org>