ofw_cpu: Make ext_resources non-optional for arm* and riscv

EXT_RESOURCES have been introduced in 12-CURRENT and all supported
releases have it enabled in their kernel config.
For RISC-V it was always enabled.

MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D33835

(cherry picked from commit 02398e581d4511bd1c5e8ffad5155f9b9684b4b8)