mtd: spi-nor: Disable chip erase for Micron n25q00.

Micron n25q00 are stacked chips, thus do not support chip erase.
>From now spi-nor framework will not send chip erase command,
instead will use sector at time erase procedure.

Signed-off-by: Marcin Krzeminski <mar.krzeminski@gmail.com>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>