IRQCHIP: bcm7120-l2: Split STB-specific logic into its own function
The BCM7xxx instances of this block (listed in the register manual as
simply "IRQ0") all have the following items in common:
- brcm,int-map-mask: for routing different bits in the L2 to different
parent IRQs
- brcm,int-fwd-mask: for hardwiring certain IRQs to bypass the L2 and
use dedicated L1 lines
- one enable/status pair (32 bits only)
Much of the driver code can be shared with BCM3380-style controllers, but
in order to do this cleanly, let's split out the BCM7xxx-specific logic
first.