irqchip/gic: Make interrupt ID 1020 invalid

The GIC has no such thing as interrupt 1020: the last valid ID is
1019, and the range 1020-1023 is reserved - 1023 indicating that
no interrupt is pending. So let's make sure we don't try to handle
this ID.

This bug has been in since the initial GIC code was introduced in
8ad68bbf ("[ARM] Add support for ARM RealView board").

Reported-by: Eric Auger <eric.auger@linaro.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>