soc/mediatek: Add second "DMA" memory region

The Linux SOF driver owns this space, but puts buffers here which the
DSP is expected to read and write.  Give it a space in the default MPU
mapping.

Note that this assumes that the DMA region is numerically above the
linkable DRAM region in the address space.  That's true for existing
devices, but likely to be broken in the future.

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