samples: add support for i.MX8ULP DSP in openamp_rsc_table

Add the dts and config overlay for imx8ulp_evk//adsp
to have the openamp_rsc_table sample working on
HiFi4 DSP from i.MX8ULP.

Since openamp_rsc_table sample uses IPM API and i.MX8ULP
has a mbox driver for messaging unit, we need a mailbox node.
This will load the ipm_mbox driver that exposes the IPM
interface while using a MBOX driver in the back-end.

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