tests/functional: Add test for imx8mp-evk board with USDHC coverage

tests/functional: Add test for imx8mp-evk board with USDHC coverage

Introduce a functional test which boots Debian 12 on the imx8mp-evk board. Since
the root filesystem resides on an SD card, the test also verifies the basic
operation of the USDHC.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-id: 20250409202630.19667-1-shentey@gmail.com
[PMM: added extra blank line as suggested by thuth;
 set timeout to 240s]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>