dt-bindings: mfd: mediatek: mt6397: Change to relative paths

Paths in dt-bindings should be relative as suggested by Lee Jones.

Suggested-By: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>