mtd: atmel-quadspi: disallow building on ebsa110

mtd: atmel-quadspi: disallow building on ebsa110

I ran into a link-time error with the atmel-quadspi driver on the
EBSA110 platform:

drivers/mtd/built-in.o: In function `atmel_qspi_run_command':
:(.text+0x1ee3c): undefined reference to `_memcpy_toio'
:(.text+0x1ee48): undefined reference to `_memcpy_fromio'

The problem is that _memcpy_toio/_memcpy_fromio are not available on
that platform, and we have to prevent building the driver there.

In case we want to backport this to older kernels: between linux-4.8
and linux-4.20, the Kconfig entry was in drivers/mtd/spi-nor/Kconfig
but had the same problem.

Link: https://lore.kernel.org/patchwork/patch/812860/
Fixes: 161aaab8a067 ("mtd: atmel-quadspi: add driver for Atmel QSPI controller")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org