d/rules: Add imx8mn_uart4 target subplatform

https://phabricator.apertis.org/T8220

d/rules: Add imx8mn_uart4 target subplatform

By default, iMX8MN uses UART2 console. However, other boards support in
Apertis (e.g. Variscite VAR-SOM-MX8M-NANO board, BSH SMM S2 boards) uses
UART4.

Add two subplatforms for imx8mn platform:
  * imx8mn: default configuration
  * imx8mn_uart4: console set to UART4

This will generate the following tree:

  $ tree usr/lib/arm-trusted-firmware/imx8mn*
  usr/lib/arm-trusted-firmware/imx8mn
  └── bl31.bin
  usr/lib/arm-trusted-firmware/imx8mn_uart4
  └── bl31.bin