Add sbbc(4), a driver for the BootBus controller found in Serengeti and
StarCat systems which provides time-of-day services for both as well as
console service for Serengeti, i.e. Sun Fire V1280. While the latter is
described with a device type of serial in the OFW device tree, it isn't
actually an UART. Nevertheless the console service is handled by uart(4)
as this allowed to re-use quite a bit of MD and MI code. Actually, this
idea is stolen from Linux which interfaces the sun4v hypervisor console
with the Linux counterpart of uart(4).