ath79: add ATH79_CPU_IRQ() macro

Remove the individual ATH79_CPU_IRQ_* constants and
use the new macro instead of those.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/4929/


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