[MTD] nandsim: enhance nandsim to allow arbitrary NAND size

A new module parameter has been added called 'overridesize',
which overrides the size that would be determined by the
ID bytes. 'overridesize' is specified in erase blocks and
as the exponent of a power of two e.g. 5 means a size of
32 erase blocks.

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>