ASoC: mt6359: fix failed to parse DT properties

Mt6359 platform device is instantiated by mfd_add_devices().  In the
case, dev->of_node is NULL so that mt6359_parse_dt() always fails to
parse the desired DT properties.

Gets the DT properties via dev->parent->of_node.

Fixes: 8061734a

 ("ASoC: mediatek: mt6359: add codec driver")
Signed-off-by: Tzung-Bi Shih <tzungbi@google.com>
Link: https://lore.kernel.org/r/20200908070044.1142644-1-tzungbi@google.com
Signed-off-by: Mark Brown <broonie@kernel.org>