ARM: dts: msm: Add PCIe endpoint node for sdxpoorwills

Add PCIe endpoint node to device tree of sdxpoorwills. PCIe core
is in endpoint mode and communicates with the PCIe root complex
on host side.

Change-Id: Ibbe9f840ac51459db7eac26b2ddb900e4d9265ce
Signed-off-by: Yan He <yanhe@codeaurora.org>