hw/arm/fsl-imx8mp: Add USDHC storage controllers

hw/arm/fsl-imx8mp: Add USDHC storage controllers

The USDHC emulation allows for running real-world images such as those generated
by Buildroot. Convert the board documentation accordingly instead of running a
Linux kernel with ephemeral storage.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Message-id: 20250223114708.1780-8-shentey@gmail.com
[PMM: drop 'static const' from usdhc_table[] for GCC 7.5]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>