mtd: spi-nor: spansion: Consider reserved bits in CFR5 register

CFR5[6] is reserved bit and must be always 1. Set it to comply with flash
requirements. While fixing SPINOR_REG_CYPRESS_CFR5V_OCT_DTR_{EN, DS}
definition, stop using magic numbers and describe the missing bit fields
in CFR5 register. This is useful for both readability and future possible
addition of Octal STR mode support.

Fixes: c3266af1 ("mtd: spi-nor: spansion: add support for Cypress Semper flash")
Cc: stable@vger.kernel.org
Reported-by: Takahiro Kuwano <Takahiro.Kuwano@infineon.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Reviewed-by: Dhruva Gole <d-gole@ti.com>
Reviewed-by: Pratyush Yadav <ptyadav@amazon.de>
Tested-by: Dhruva Gole <d-gole@ti.com>
Link: https://lore.kernel.org/linux-mtd/20230110164703.83413-1-tudor.ambarus@linaro.org