Initial support for Raspberry Pi 2/3. All the hard work done by patrick@, I

Initial support for Raspberry Pi 2/3.  All the hard work done by patrick@, I
just cleaned things up a bit.  Any bugs introduced in that process are
entirely mine.

This doesn't work yet.  But when it does, you'll need recent firmware from
the Raspberry Pi Foundation git repository at

  https://github.com/raspberrypi/firmware

The device tree for the Raspberry Pi is somewhat in flux as bits and pieces
to support the Raspberry Pi 2 and 3 are committed to the mainline Linux
kernel.