mtd: spi-nor: Fix whole chip erasing for stacked chips.

Currently it is possible to disable chip erase for spi-nor driver.
Some modern stacked (multi die) flash chips do not support chip
erase opcode at all but spi-nor framework needs to cope with them too.
This commit extends existing functionality to allow disable
chip erase for a single flash chip.

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