mtd: spi-nor-core: Make CFRx reg fields generic

Cypress defines two flavors of configuration registers, volatile and
non volatile, and both use the same bit fields. Rename the bitfields in
the configuration registers so that they can be used for both flavors.

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>