riscv: dts: Add QSPI NAND device node

Add QSPI NAND device node to the Microchip PolarFire SoC
Icicle kit device tree.

The Winbond NAND flash memory can be connected to the
Icicle Kit by using the Mikroe Flash 5 click board and
the Pi 3 Click shield.

Signed-off-by: Padmarao Begari <padmarao.begari@microchip.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Rick Chen <rick@andestech.com>