samples: drivers: spi_flash: add support for arty_a7_arm_designstart_m3

Add support for running the JEDEC SPI NOR flash sample on the ARM
Cortex-M3 DesignStart FPGA reference implementation.

Special care is needed to avoid overwriting part of the FPGA
configuration bitstream, which is stored in the onboard QSPI flash.

Signed-off-by: Henrik Brix Andersen <henrik@brixandersen.dk>