Fall back to use the GICR address from the generic interrupt struct

When there is no ACPI redistributor sub-table in the MADT we need to
fall back to use the GICR base address from the GIC CPU interface
structure.

Handle this fallback when adding memory to the device and when counting
the number of redistributors.

PR: 251171
Reported by: Andrey Fesenko <f0andrey_gmail.com>
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D27247