dt-bindings: i2c-mt6577: Rename file to reflect bindings

The i2c-mt6577.txt actually holds the bindings for all mediatek supported i2c
controller. Change the name to i2c.mtk.txt to reflect that.
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Wolfram Sang <wsa@the-dreams.de>