MIPS: ath79: add PCI_AR724X Kconfig symbol

The AR724X specific PCI code can be used for the
AR934X SoCs, however it can be selected only if
SOC_AR724X is set.

Introduce a new Kconfig symbol in order to be able
to use the code for AR934X as well.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Acked-by: Luis R. Rodriguez <mcgrof@qca.qualcomm.com>
Cc: linux-mips@linux-mips.org
Cc: mcgrof@infradead.org
Patchwork: https://patchwork.linux-mips.org/patch/3514/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>