bcm2838_pci.c: Respect DMA limits of controller.

bcm2838_pci.c: Respect DMA limits of controller.

Fixes for Raspberry Pi 4B PCIe / USB:
- Pass through a DMA tag for the controller.
- In theory the controller can access the lower 3 GB, but testing found
  that unreliable. OpenBSD also restricts DMA to the lowest 960 MiB.
- Rename some constants to be a bit more meaningful.

Submitted by:	Robert Crowston, crowston at protonmail.com
Reviewed by:	mkarels, outside reviewers
Differential Revision:	https://reviews.freebsd.org/D26344