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