drivers: xilinx_spi: Probe fifo_depth at runtime

If the fifo-size DT parameter is not provided then probe the
controller's fifo depth at runtime. This is ported from a patch
in the Linux Xilinx SPI driver.

Signed-off-by: Mayuresh Chitale <mchitale@ventanamicro.com>
Reviewed-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/1422029330-10971-5-git-send-email-ricardo.ribalda@gmail.com


Tested-by: Love Kumar <love.kumar@amd.com>
Link: https://lore.kernel.org/r/20231116164336.140171-4-mchitale@ventanamicro.com


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