mtd: spi-nor-core: 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 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: ea9a22f7 ("mtd: spi-nor-core: Add support for Cypress Semper flash")
Suggested-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Signed-off-by: Takahiro Kuwano <Takahiro.Kuwano@infineon.com>
Reviewed-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Reviewed-by: Dhruva Gole <d-gole@ti.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>