arm64: Generate HYP timer interrupt specifiers

FEAT_VHE introduced a non-secure EL2 virtual timer, along with its
interrupt line. Consequently the arch timer DT binding introduced a fifth
interrupt to communicate this interrupt number.

Refactor the interrupts property generation code to deal with a variable
number of interrupts, and forward five interrupts instead of four in case
nested virt is enabled.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Sascha Bischoff <sascha.bischoff@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>