drivers: spi: Add support for Polarfire SOC SPI

Add driver for the Microchip Polarfire SOC MSS SPI controller.
The interrupts of the MSS SPI 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>