arm: Add a missing interrupt to the generic timer

The ACPI generic timer attachment added 3 interrupts, but missed the
hypervisor physical interrupt. As the field is present in all versions
of the GTDT ACPI table and isn't an optional interrupts we can add it
to the interrupts provided to the driver.

Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D43974

(cherry picked from commit f715e95461c9264913a65e2dc93c291b7e318d2e)