msm: pcie: get PCIe PHY power down register offset from DT

PCIe PHY power down register offset varies on each chipset.
Therefore, move this offset to devicetree. This register is
needed to control the PCIe PHY low power mode.

Change-Id: Iec69c2372b13c9bb459b24ecb5ce001319213b90
Signed-off-by: Tony Truong <truong@codeaurora.org>