mtd: fixup CFI on ixp4xx

drivers/mtd/maps/ixp4xx.c requires MTD_CFI_BE_BYTE_SWAP to be set
in order to compile.

drivers/mtd/maps/ixp4xx.c:57:4: error: #error CONFIG_MTD_CFI_BE_BYTE_SWAP required

This patch avoids the #error output by enforcing the policy in
Kconfig. Not sure if this is the right approach, but it helps doing
randconfig builds.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210927141045.1597593-1-arnd@kernel.org