ARM64: dts: bcm2835: Fix bcm2837 compatible string

bcm2837-rpi-3-b.dts, its only in-tree user, was overriding it as
"brcm,bcm2837" already.

Fixes: 9d56c22a ("ARM: bcm2835: Add devicetree for the Raspberry Pi 3.")
Cc: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Eric Anholt <eric@anholt.net>