d/target: Use imx8mn_uart4 ATF binary for Variscite and BSH boards
https://phabricator.apertis.org/T8220
Depends on arm-trusted-firmware!8 (merged)
Apertis arm-trusted-firmware package recently added two subplatforms for the imx8mn platform:
- imx8mn: default configuration
- imx8mn_uart4: console set to UART4
Variscite VAR-SOM-MX8M-NANO and BSH SMM S2 boards uses UART4 instead of UART2 for console output. Pick the right ATF binary.