hw/ppc/e500: Implement pflash handling

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>