irqchip/exiu: Add support for Socionext Synquacer EXIU controller

The Socionext Synquacer SoC has an external interrupt unit (EXIU)
that forwards a block of 32 configurable input lines to 32 adjacent
level-high type GICv3 SPIs.

The EXIU has per-interrupt level/edge and polarity controls, and
mask bits that keep the outgoing lines de-asserted, even though
the controller may still latch interrupt conditions that occur
while the line is masked.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>