bcm2835: push address mapping conversion for DMA/mailbox to runtime

We could maintain the static conversions for the !AArch64 Raspberry Pis, but
I'm not sure it's worth it -- we'll traverse the platform list exactly once
(of which there are only two for armv7), then every conversion there-after
traverses the memory map listing of which there are at-most two entries for
these boards: sdram and peripheral space.

Detecting this at runtime is necessary for the AArch64 SOC, though, because
of the distinct IO windows being otherwise not discernible just from support
compiled into the kernel. We currently select the correct window based on
/compatible in the FDT.

We also use a similar mechanism to describe the DMA restrictions- the RPi 4
can have up to 4GB of RAM while the DMA controller and mailbox mechanism can
technically, kind of, only access the lowest 1GB. See the comment in
bcm2835_vcbus.h for a fun description/clarification of this.

Differential Revision: https://reviews.freebsd.org/D22301