arm: generic_timer: use interrupt-names when available

Offsets for all of thse can be a bit complicated as not all interrupts
will be present, only phys and virt are actually required, and sec-phys
could optionally be specified before phys. Push idx/name pairs into
a new config struct and maintain the old indices while still getting the
correct timers.

Split fdt/acpi attach out independently and allocate interrupts before
we head into the common attach(). The secure physical timer is also
optional there, so mark it so to avoid erroring out if we run into
problems.

Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D38911

(cherry picked from commit 91b2da13702fb3cfb40a3219feed6e5af651039d)