bcm2711: Retain support for old dtbs

The recent series switching to bcm2711 as the DT identifier broke Pis
running with old DTBs. Add some bcm2838 compatible strings as a
temporary measure, at least until the next full Raspbian image with
bcm2711 DTBs.

See: https://github.com/raspberrypi/linux/pull/3244



Signed-off-by: Phil Elwell <phil@raspberrypi.org>