dt-bindings: soc: bcm: bcm2835-pm: Introduce reg-names

Anticipating the introduction of BCM2711, of which we'll need to support
its new RPiVid ASB, introduce reg-names into bcm2835-pm's binding. This
will help to have a consistent mapping between resources and their
meaning.

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Peter Robinson <pbrobinson@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>