PCI/xilinx: Depend on OF 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, which is the only true dependency.

Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>
[hch: switch to OF instead of OF_PCI now that the latter is gone]
Signed-off-by: Christoph Hellwig <hch@lst.de>
[lorenzo.pieralisi@arm.com: trimmed the commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>