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

Introduce n_regions in spi_nor_erase_map structure and remove
SNOR_LAST_REGION flag. Loop logics that depend on the flag are also
reworked to use n_regions as loop condition.

Signed-off-by: Takahiro Kuwano <Takahiro.Kuwano@infineon.com>
Suggested-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Suggested-by: Michael Walle <mwalle@kernel.org>
Reviewed-by: Michael Walle <mwalle@kernel.org>
Link: https://lore.kernel.org/r/eded84294bd81e966d6f423e578fc2cfb9a4a5b6.1708404584.git.Takahiro.Kuwano@infineon.com
[ta: update spi_nor_init_erase_cmd_list() and break the for loop sooner.]
Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>