ANDROID: GKI: add built-in PCIE_DW_PLAT_EP

To build an EP driver which is based on PCIe designware as module, we
need to get some symbols from pcie-designware_ep.c. To select the
source from GKI kernel, we should enable PCIE_DW_PLAT_EP driver as
prebuilt.

Bug: 167931570

Signed-off-by: Chanho Park <chanho61.park@samsung.com>
Change-Id: I12f796cb35794e29d1fa425d26e2c9ee2c855da2