dts: bcm2712-rpi-5-b: Create some dummy nodes

The kernel now treats multiple fragments targeting the same node as an
error. For this reason, it is important that labels created just for
compatibility with other systems (e.g. i2c0if and i2c0mux) are
attached to unique nodes, not just tacked onto existing nodes.

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