dt-bindings: serial: Add compatible for Mediatek MT7986

This commit adds dt-binding documentation of uart for Mediatek MT7986 SoC
Platform.

Signed-off-by: Sam Shih <sam.shih@mediatek.com>
Link: https://lore.kernel.org/r/20210726071439.14248-10-sam.shih@mediatek.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>