MIPS: ath79: add a common PCI registration function

The current code unconditionally registers the AR724X
specific PCI controller, even if the kernel is running
on a different SoC.

Add a common function for PCI controller registration,
and only register the AR724X PCI controller if the kernel
is running on an AR724X SoC.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/3488/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>