dt-bindings: serial: 8250: Add MediaTek BTIF controller bindings

Document the devicetree bindings in 8250.txt for MediaTek BTIF
controller which could be found on MT7622 and MT7623 SoC.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>