bsh: imx8mn-smm-s2pro: Set u-boot prompt to "U-Boot>"
Task: https://phabricator.apertis.org/T8220
The boot procedure on LAVA is having problem with the u-boot prompt being set to ">" on BSH SMM S2 devices. Let's set it to "U-Boot>" so it matches what other supported boards do, e.g. RPi4.