tests: drivers: spi: loopback: add support for arty_a7_arm_designstart_m1

Add support for running the SPI loopback driver test on the ARM
Cortex-M1 DesignStart FPGA reference design.

Since Xilinx AXI Quad SPI IP only supports loopback mode when configured
for single line SPI width, we utilise the (normally disabled) single SPI
instance going to the optional V2C DAPLINK shield SD card slot for
testing purposes.

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