dt-bindings: mediatek: mt8192: add mt8192-mt6358-rt1015-rt5682 document

This patch adds document for the machine board with mt6358, rt1015
and rt5682.

Signed-off-by: Jiaxin Yu <jiaxin.yu@mediatek.com>
Link: https://lore.kernel.org/r/1604390378-23993-10-git-send-email-jiaxin.yu@mediatek.com


Signed-off-by: Mark Brown <broonie@kernel.org>