mtd: spi-nor: core: rework struct spi_nor_erase_region

mtd: spi-nor: core: rework struct spi_nor_erase_region

Encoding bitmask flags into offset worsen the code readability. The
erase type mask and flags should be stored in dedicated members. Also,
erase_map.uniform_erase_type can be removed as it is redundant.

Signed-off-by: Takahiro Kuwano <Takahiro.Kuwano@infineon.com>
Suggested-by: Michael Walle <mwalle@kernel.org>
Reviewed-by: Michael Walle <mwalle@kernel.org>
Link: https://lore.kernel.org/r/8e5e9e4081ed9f16ea9dce30693304a4b54d19b1.1708404584.git.Takahiro.Kuwano@infineon.com
[ta: remove spi_nor_region_end()]
Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>