ARM: dts: bcm2711: Add firmware clocks node

Now that we have a clock driver for the clocks exposed by the firmware,
let's add the device tree nodes for it.

Tested-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Link: https://lore.kernel.org/r/9a6f113140b3115150bfb18ecb248a48d58562cf.1592210452.git-series.maxime@cerno.tech