irqchip: Add sl28cpld interrupt controller support

Add support for the interrupt controller inside the sl28 CPLD management
controller.

The interrupt controller can handle at most 8 interrupts and is really
simplistic and consists only of an interrupt mask and an interrupt
pending register.

Signed-off-by: Michael Walle <michael@walle.cc>
Acked-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>