mtd: sh_flctl: Group sector accesses into a single transfer

When we use hardware ecc, the flctl is run in so-called "sector access
mode". We can bundle 4 sector accesses when using 2KiB page sizes to read
a whole page at once and speed up things.

Signed-off-by: Bastian Hecht <hechtb@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>