dt-bindings: remoteproc: imx_rproc: support i.MX8ULP

Add i.MX8ULP remote processor(Cortex-M33) compatible string

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Link: https://lore.kernel.org/r/20210622060148.18411-2-peng.fan@oss.nxp.com


Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>