PCI: controller: PCI_IXP4XX should depend on ARCH_IXP4XX

The Intel IXP4xx PCI controller is only present on Intel IXP4xx
XScale-based network processor SoCs.

Add a dependency on ARCH_IXP4XX, to prevent asking the user about this
driver when configuring a kernel without support for the XScale
processor family.

Link: https://lore.kernel.org/r/6a88e55fe58fc280f4ff1ca83c154e4895b6dcbf.1624972789.git.geert+renesas@glider.be
Fixes: f7821b49

 ("PCI: ixp4xx: Add a new driver for IXP4xx")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
[lorenzo.pieralisi@arm.com: commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>