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