The SMMU_IRQ_CTRL had been made optionally writeable by a
prior patch [1] even if interrupts were not supported in
the SMMUv3 model.
As we are partially enabling IRQ support, we remove this option
and we make the SMMU_IRQ_CTRL always writeable
[1]: https://gem5-review.googlesource.com/c/public/gem5/+/38555
Change-Id: Ie1f9458d583a5d8bcbe450c3e88bda6b3c53cf10
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>