irqchip/brcmstb-l2: Abstract register accesses

Added register block offsets to the brcmstb_l2_intc_data structure
for the status and mask registers to support reading the active
interupts in an abstracted way.  It seems like an irq_chip method
should have been provided for this, but it's not there yet.

Abstracted the implementation of the handler, suspend, and resume
functions to not use any hard coded register offsets.

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Doug Berger <opendmb@gmail.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>