samples: add support for i.MX8QM and i.MX8QXP DSP core in openamp_rsc_table

Add the dts and config overlay for the two boards in order
to have the openamp_rsc_table sample working on
HiFi4 DSP from i.MX8QM and i.MX8QXP.

Since these two are similar, use a snippet.

Therefore, to compile the sample we use:
west build -p -b imx8qm_mek/mimx8qm6/adsp -S nxp-openamp-imx8-adsp
-s samples/subsys/ipc/openamp_rsc_table/

Signed-off-by: Iuliana Prodan <iuliana.prodan@nxp.com>