soc/mtk_adsp: Remove "msg" API

This is a feature of the 8195 DSP only, which is used only vestigially
by SOF to store data that nothing reads.  The Linux kernel on the
other side uses a shared driver for all 81xx devices, which does not
expose the feature.  It seems to work, but it's not worth maintaining
a driver in tree for legacy hardware that will never use it.

Signed-off-by: Andy Ross <andyross@google.com>