This allows to save and restore the content of the PRAM.
It may be useful if we want to check the configuration or to change it.

The backend is added using mtd interface, for instance:

    ... -drive file=pram.img,format=raw,if=mtd ...

where pram.img is the file where the data will be stored, its size must
be 256 bytes.
Signed-off-by: NLaurent Vivier <laurent@vivier.eu>
Message-Id: <20191219201439.84804-3-laurent@vivier.eu>