BCM2708: Use upstream interrupt driver on all Pi1's

Although the aim is to delete the old armctrl driver, that can't happen
while non-DT configurations are still supported. This commit enables
use of the new FIQ-enabled upstream irqchip driver when DT is enabled
on all BCM2835-based RPi platforms (Models A, B, A+, B+ & CM).

BCM2836-based platforms (Pi 2) will get the same treatment at a later
date, unless non-DT support has already been withdrawn.