dt-bindings: net: dsa: add new MT7530 binding to support MT7621

Add devicetree binding to support the compatible mt7530 switch as used
in the MediaTek MT7621 SoC.

Signed-off-by: Greg Ungerer <gerg@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Sean Wang <sean.wang@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>