board: imx93_var_som: Probe ELE MU

Probing the MU is needed to prevent this error in the SPL:

ele dev is not initialized
Authenticate container hdr failed, return -19, resp 0x0
IND = INVALID

ele dev is not initialized
Error: release container failed, resp 0x0!
IND = INVALID

SPL: failed to boot from all boot devices

Signed-off-by: Mathieu Othacehe <othacehe@gnu.org>