mtd: spi-nor: core: get rid of SNOR_OVERLAID_REGION flag

Only SNOR_OVERLAID_REGION is defined for flags in the spi_nor_erase_region
structure. It can be replaced by a boolean parameter.

Signed-off-by: Takahiro Kuwano <Takahiro.Kuwano@infineon.com>
Reviewed-by: Michael Walle <mwalle@kernel.org>
Link: https://lore.kernel.org/r/076416d5dc8328dec72d31db12b9bec96bf0ac66.1708404584.git.Takahiro.Kuwano@infineon.com
Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>