soc: mediatek: mmsys: Use devm_platform_ioremap_resource()

For the common platform_get_resource()+devm_platform_ioremap() combination,
there is a helper, so use it and make the code a bit more compact.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Link: https://lore.kernel.org/r/20201006193320.405529-3-enric.balletbo@collabora.com


Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>