hw/ppc/e500: Implement pflash handling

Allows e500 boards to have their root file system reside on flash using
only builtin devices located in the eLBC memory region.

Note that the flash memory area is only created when a -pflash argument is
given, and that the size is determined by the given file. The idea is to
put users into control.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20221018210146.193159-6-shentey@gmail.com>
[danielhb: use memory_region_size() in mmio_size]
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>

docs/system/ppc/ppce500.rst

@@ -165,3 +165,18 @@ if “-device eTSEC” is given to QEMU:
 .. code-block:: bash
 
   -netdev tap,ifname=tap0,script=no,downscript=no,id=net0 -device eTSEC,netdev=net0
+
+Root file system on flash drive
+-------------------------------
+
+Rather than using a root file system on ram disk, it is possible to have it on
+CFI flash. Given an ext2 image whose size must be a power of two, it can be used
+as follows:
+
+.. code-block:: bash
+
+  $ qemu-system-ppc64 -M ppce500 -cpu e500mc -smp 4 -m 2G \
+      -display none -serial stdio \
+      -kernel vmlinux \
+      -drive if=pflash,file=/path/to/rootfs.ext2,format=raw \
+      -append "rootwait root=/dev/mtdblock0"