irqchip/sun4i: Add a struct to hold global variables

In order to support different chips, IC specific data should be hold in
a struct. This patch moves irq_base and irq_domain global variables to
struct.

Signed-off-by: Mesih Kilinc <mesihkilinc@gmail.com>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>