[MTD] nandsim: add partition capability to nandsim

Enhance nandsim to be able to create more than 1 partition.
A new module parameter 'parts' may be used to specify partition
sizes.

Signed-off-by: Adrian Hunter <ext-adrian.hunter@nokia.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>