device-types: rk3588-rock-5b: Update for mainline U-Boot

U-Boot v2023.07 gained support for the Rock 5B (including booting from the onboard ethernet port); until now we were using a downstream fork which required a number of workarounds to boot using a USB ethernet dongle. There were also a number of device template changes to work with the downstream U-Boot (e.g. changing the bootloader interrupt prompt/character).

Update the device template for rk3588-rock-5b to utilise mainline U-Boot and synchronise variables with other Rockchip device template (rk3399-rock-pi-4b.jinja2).