ARM: dts: msm: enable PCIe EP mode for SDXPOORWILLS

Enable PCIe EP (endpoint) mode and disable PCIe RC (root complex) mode
for RmNet Tethering use case on SDXPOORWILLS.

Change-Id: I8a77159fb3e11fd99f09697b96bfa480ca0763a7
Signed-off-by: Yan He <yanhe@codeaurora.org>