mtd: OneNAND: multiblock erase support

Add support for multiblock erase command. OneNANDs (excluding Flex-OneNAND)
are capable of simultaneous erase of up to 64 eraseblocks which is much faster.

This changes the erase requests for regions covering multiple eraseblocks
to be performed using multiblock erase.

Signed-off-by: Mika Korhonen <ext-mika.2.korhonen@nokia.com>
Reviewed-by: Adrian Hunter <adrian.hunter@nokia.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>