dt-bindings: simplefb: Document naming scheme for pre-populated simplefb nodes

Since we advice to use pre-populated simeplfb nodes in dt files, where the
firmware then only needs to fill in the mode info + address and enable it,
we should also specify how these pre-populated nodes should be named so
that the firmware can find and enable the right one.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>