spi: atmel-quadspi: Depend on SPI_MEM

spi: atmel-quadspi: Depend on SPI_MEM

Most other spi-mem drivers also depend on SPI_MEM.  Fixes this build
error:

    arm-v5te-linux-gnueabi-ld.bfd: drivers/spi/atmel-quadspi.o: in function `atmel_qspi_supports_op':
    /mnt/data/adahl/src/u-boot/drivers/spi/atmel-quadspi.c:460: undefined reference to `spi_mem_default_supports_op'
    make[1]: *** [/mnt/data/adahl/src/u-boot/Makefile:1821: u-boot] Error 1

Signed-off-by: Alexander Dahl <ada@thorsis.com>
Reviewed-by: Tom Rini <trini@konsulko.com>