ARM: dts: bcm2835: Move rng definition to common location

BCM2711 inherits from BCM283X, but has an incompatible HWRNG. Move this
node to bcm2835-common.dtsi, so that BCM2711 can define its own.

Signed-off-by: Stephen Brennan <stephen@brennan.io>
Acked-by: Stefan Wahren <wahrenst@gmx.net>
Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>