MIPS: ath79: move global PCI defines into a common header

The constants will be used by a subsequent patch.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/4907/


Signed-off-by: John Crispin <blogic@openwrt.org>