fpga: altera-fpga2sdram: remove unused struct 'prop_map'

'prop_map' has been unused since the original
commit e5f8efa5 ("ARM: socfpga: fpga bridge driver support").

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Reviewed-by: Russ Weight <russ.weight@linux.dev>
Acked-by: Xu Yilun <yilun.xu@intel.com>
Link: https://lore.kernel.org/r/20240530205245.125513-1-linux@treblig.org


Signed-off-by: Xu Yilun <yilun.xu@linux.intel.com>