Create macros for the Arm timer IRQ indexes

Rather than hard coding these values use a macro to document which
interrupt is being used.

Reviewed by: emaste
Obtained from: https://github.com/FreeBSD-UPB/freebsd-src (earlier version)
Sponsored by: Innovate UK
Sponsored by: The FreeBSD Foundation
Sponsored by: University Politehnica of Bucharest
Differential Revision: https://reviews.freebsd.org/D36600