drivers: xilinx_spi: Add xfer callback

Add the xfer callback which is used by the MMC_SPI driver and generally by
the dm_spi_xfer callback.

Signed-off-by: Mayuresh Chitale <mchitale@ventanamicro.com>
Tested-by: Love Kumar <love.kumar@amd.com>
Link: https://lore.kernel.org/r/20231116164336.140171-3-mchitale@ventanamicro.com


Signed-off-by: Michal Simek <michal.simek@amd.com>