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

sys/arm/arm/nexus.c

@@ -34,9 +34,7 @@
  * attachment point for both processors and buses, and to manage
  * resources which are common to all of them.  In particular,
  * this code implements the core resource managers for interrupt
- * requests, DMA requests (which rightfully should be a part of the
- * ISA code but it's easier to do it here for now), I/O port addresses,
- * and I/O memory address space.
+ * requests and I/O memory address space.
  */
 
 #include "opt_platform.h"

sys/arm64/arm64/nexus.c

@@ -34,9 +34,7 @@
  * attachment point for both processors and buses, and to manage
  * resources which are common to all of them.  In particular,
  * this code implements the core resource managers for interrupt
- * requests, DMA requests (which rightfully should be a part of the
- * ISA code but it's easier to do it here for now), I/O port addresses,
- * and I/O memory address space.
+ * requests and I/O memory address space.
  */
 
 #include "opt_acpi.h"

sys/riscv/riscv/nexus.c

@@ -34,9 +34,7 @@
  * attachment point for both processors and buses, and to manage
  * resources which are common to all of them.  In particular,
  * this code implements the core resource managers for interrupt
- * requests, DMA requests (which rightfully should be a part of the
- * ISA code but it's easier to do it here for now), I/O port addresses,
- * and I/O memory address space.
+ * requests and I/O memory address space.
  */
 #include "opt_platform.h"