raspi: Raspberry Pi 3 support

raspi: Raspberry Pi 3 support

This patch adds Raspberry Pi 3 support to hw/arm/raspi.c. The
differences to Pi 2 are:

 - Firmware address
 - Board ID
 - Board revision

The CPU is different too, but that's going to be configured as part of
the machine default CPU when we introduce a new machine type.

The patch was written from scratch by me but the logic is similar to
Zoltán Baldaszti's previous work, which I used as a reference (with
permission from the author):

  https://github.com/bztsrc/qemu-raspi3

Signed-off-by: Pekka Enberg <penberg@iki.fi>
[PMM: fixed trailing whitespace on one line]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>