spi: bcm-qspi: shut up warning about cfi header inclusion

When CONFIG_MTD_CFI is disabled, we get a warning for this spi driver:

include/linux/mtd/cfi.h:76:2: #warning No CONFIG_MTD_CFI_Ix selected. No NOR chip support can work. [-Werror=cpp]

The problem here is a layering violation that was fixed in mainline kernels with
a larger rework in commit 054e532f8f90 ("spi: bcm-qspi: Remove hardcoded settings
and spi-nor.h dependency"). We can't really backport that to stable kernels, so
this just adds a Kconfig dependency to make it either build cleanly or force it
to be disabled.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>