drivers: spi_flash_at45: Fix erasing of first two sectors

Most available AT45 flash chips have their first two sectors shorter
than the consecutive ones. Usually, the first sector is marked as 0a
and has its size equal to eight pages (one block) and the second one
(usually 0b) is the complement to the size of a regular sector.
This commits modifies the driver so that erasing of these first two
sectors is performed correctly. This modified behavior is configurable
with a new DT property so that it is still possible to also use legacy
AT45 chips that do not feature such sector split. Such legacy chips
usually also do not support the chip erase and sector erase commands,
so two more DT properties are introduced to cover that.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>