drivers: spi: Add support for Polarfire SOC QSPI

This patch adds driver for the Microchip Polarfire SOC MSS QSPI
controller.
The interrupts of the MSS QSPI are routed through PLIC(Platform level
interrupt controller).

Tested with generic spi-nor flash driver(spi_flash) with both Fixed
flash configuration and Read flash parameters at runtime(using SFDP).

Signed-off-by: Naga Sureshkumar Relli <nagasuresh.relli@microchip.com>