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>

docs/system/arm/imx8mp-evk.rst

@@ -12,6 +12,7 @@ The ``imx8mp-evk`` machine implements the following devices:
  * Up to 4 Cortex-A53 cores
  * Generic Interrupt Controller (GICv3)
  * 4 UARTs
+ * 3 USDHC Storage Controllers
  * Secure Non-Volatile Storage (SNVS) including an RTC
  * Clock Tree
 
@@ -26,18 +27,23 @@ Direct Linux Kernel Boot
 
 Probably the easiest way to get started with a whole Linux system on the machine
 is to generate an image with Buildroot. Version 2024.11.1 is tested at the time
-of writing and involves two steps. First run the following commands in the
+of writing and involves three steps. First run the following commands in the
 toplevel directory of the Buildroot source tree:
 
 .. code-block:: bash
 
-  $ echo "BR2_TARGET_ROOTFS_CPIO=y" >> configs/freescale_imx8mpevk_defconfig
   $ make freescale_imx8mpevk_defconfig
   $ make
 
 Once finished successfully there is an ``output/image`` subfolder. Navigate into
-it and patch the device tree with the following commands which will remove the
-``cpu-idle-states`` properties from CPU nodes:
+it and resize the SD card image to a power of two:
+
+.. code-block:: bash
+
+  $ qemu-img resize sdcard.img 256M
+
+Finally, the device tree needs to be patched with the following commands which
+will remove the ``cpu-idle-states`` properties from CPU nodes:
 
 .. code-block:: bash
 
@@ -52,5 +58,5 @@ Now that everything is prepared the machine can be started as follows:
       -display none -serial null -serial stdio \
       -kernel Image \
       -dtb imx8mp-evk-patched.dtb \
-      -initrd rootfs.cpio \
-      -append "root=/dev/ram"
+      -append "root=/dev/mmcblk2p2" \
+      -drive file=sdcard.img,if=sd,bus=2,format=raw,id=mmcblk2