spi/xilinx: Fix compile error

Commit 58f9b0b0, "of: eliminate
of_device->node and dev_archdata->{of,prom}_node" changed the location
of the device_node pointer.  Most drivers were converted to the new
location, but the xilinx_spi_of driver was missed and now fails to
compile.

This patch fixes up the xilinx_spi_of driver to use the new location.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>