dt-bindings: net: adin1110: Add docs

Add bindings for the ADIN1110/2111 MAC-PHY/SWITCH.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandru Tachici <alexandru.tachici@analog.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>