boards: nxp: Rename mimxrt685_evk to mimxrt685_evk_mimxrt685s_cm33

In preparation for adding AMP support for i.MX RT6xx family we need
to rename existing cm33 support files to more specific names.

e.g mimxrt685_evk.dts -> mimxrt685_evk_mimxrt685s_cm33.dts

This will allow us to later add support for Cadence DSP found on i.MX
RT6xx series.

Signed-off-by: Daniel Baluta <daniel.baluta@nxp.com>