arm, arm64, riscv: adjust top-level nexus comment

These platforms don't manage resources for DMA request lines or I/O
ports, this is specific to x86. Remove the references from the comments.

Reviewed by: imp, jhb
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D32358

(cherry picked from commit 17f790f49f5879ae9888b99fdbf857010ec13f9c)