[POWERPC] Xilinx: hwicap: update booting-without-of.txt

The ICAP device in Xilinx FPGAs differs slightly between different
FPGAs.  The driver needs an additional attribute in the device tree to
distinguish this.

Signed-off-by: Stephen Neuendorffer <stephen.neuendorffer@xilinx.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>