net/wifi-firmware-kmod: enable riscv64 for wireless firmware
In [1] we got a report that iwlwifi(4) works on riscv64, so
start enabling firmware distribution for that architecture.
Given we no longer build anything (no more .ko files) this
should not be a problem.
Sponsored by: The FreeBSD Foundation
Reviewed by: jrm
Differential Revision: https://reviews.freebsd.org/D54689