ARM: dts: add endpoint's GPIO to PCIe node in DT for MSM8996

Some clients require an additional GPIO to be enabled
for successful PCIe link training. Add the needed GPIO
to PCIe node in devicetree for MSM8996.

Change-Id: I131416dee4103ace516e75d935d5ac9994f4ba9a
Signed-off-by: Tony Truong <truong@codeaurora.org>