irqchip/armada-370-xp: Pass around the driver private structure

In continuation of converting the driver to modern style, drop the
global pointer to the driver private structure and instead pass it
around the functions and callbacks, wherever possible. (There are 3
cases where it is not possible: mpic_cascaded_starting_cpu() and the
syscore operations mpic_suspend() and mpic_resume()).

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