boards: riscv: mpfs_icicle: enable system-controller spi flash

Microchip's PolarFire SoC interface with on-board spi
nor flash via system controller. This on-board spi nor flash can be
used to store FPGA design bitstream's.

Signed-off-by: Harshit Agarwal <harshit.agarwal@microchip.com>