pcie-xilinx: Depend on OF_PCI instead of the ARCH

There isn't a hard dependency of the Xilinx AXI-PCIe host bridge on any
architecture.  For example: at SiFive we map RISC-V cores to Xilinx
FPGAs and connect the Xilinx IP via a TileLink adapter, so the RISC-V
Linux port will need to be able to enable PCIE_XILINX in order to have
PCIe support.

This patch decouples the PCIE_XILINX support from ARCH.  Instead it just
depends on OF_PCI, which I believe is the only true dependency.

Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>