overlays: Standardise on compatible="brcm,bcm2835"

Curb the proliferation of compatible string combinations by
standardising on "brcm,bcm2835" to denote BCM2835 and its descendants.

As nothing in the firmware or kernel is checking overlay compatible
strings, this should be a purely cosmetic change.

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