Add a driver to talk to the Raspberry Pi firmware

Communicating with the Raspberry Pi firmware is currently handled by each
driver calling into the mbox driver, however the device tree is structured
such that they should be calling into a firmware driver.

Add a driver for this node with an interface to communicate to the firmware
via the mbox interface.

There is a sysctl to get the firmware revision. This is a unix date so can
be parsed with:

root@generic:~ # date -j -f '%s' sysctl -n dev.bcm2835_firmware.0.revision
Tue Nov 19 16:40:28 UTC 2019

Reviewed by: manu
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D25572