samples: Add i.MX RTxxx AMP samples

Add common structure for RTxxx AMP samples. Add amp_blink, amp_mbox,
amp_audio_output and amp_audio_loopback samples.

These samples are now configured only for the mimxrt685_evk, but are
intended to support other NXP microcontrollers with Xtensa-family cores
at a later date.

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