msm: ep_pcie: Add support to control pipe clock mux

Toggle between tcxo and phy pipe clock as part of
LPM sequence. Switch the source to XO before disabling
PCIe gdsc to avoid PCIe gdsc to be stuck until source
is switched to XO.

Change-Id: Ie2044da622962ff316319e7891974621e5c0ee92
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>