hw/intc/arm_gicv3: Add new gicv3_intid_is_special() function

hw/intc/arm_gicv3: Add new gicv3_intid_is_special() function

The GICv3/v4 pseudocode has a function IsSpecial() which returns true
if passed a "special" interrupt ID number (anything between 1020 and
1023 inclusive).  We open-code this condition in a couple of places,
so abstract it out into a new function gicv3_intid_is_special().

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>