net/macb: bindings doc: Merge cdns-emac to macb

Merge two bindings for the same driver to together.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Moritz Fischer <moritz.fischer@ettus.com>
Signed-off-by: Rob Herring <robh@kernel.org>