mtd: nandsim: convert pages_written[] to bitmap

nandsim.pages_written[] is the array of unsigned char which is indexed
by the page number and used for identifying which pages have been written
when cache_file is used.  Each entry holds 0 (not written) or 1 (written),
so it can be converted to bitmap.  This reduces the allocation size of
pages_written[] by 1/8.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>