samples: i2s_codec: Move audio slab to uncached

Move mem_slab (audio queue) to an uncached region with __nocache.

On platforms with data cache (such as platforms with the Cortex M7
core), it's desirable to have the audio data in an uncached region, so
that all data is committed in the selected location before DMA transfers
are triggered.

Playback problems observed on mimxrt1170_evk/mimxrt1176/cm7 before this
was done.

Signed-off-by: Vit Stanicek <vit.stanicek@nxp.com>