spi: coreqspi: add xfer function for PolarFire SoC

spi: coreqspi: add xfer function for PolarFire SoC

Add xfer function to PolarFire SoC coreqspi driver. The read and write
operations are limited to one byte at a time instead of four as CMD18
(multiple block read) reads garbage when four byte ops are enabled.

Signed-off-by: Eoin Dickson <eoin.dickson@microchip.com>
Acked-by: Leo Yu-Chi Liang <ycliang@andestech.com>