irqchip/armada-370-xp: Put static variables into driver private structure

In preparation for converting the driver to modern style put all the
interrupt controller private static variables into driver private
structure.

Access to these variables changes as:
  main_int_base		mpic->base
  per_cpu_int_base	mpic->per_cpu
  mpic_domain		mpic->domain
  parent_irq		mpic->parent_irq
  ...

Signed-off-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>