powerpc/xsysace: add compatible string for non-ipcore instance

Add "xlnx,sysace" compatible string to the of_platform binding
table.  Platforms which have the SysACE chip on board (e.g.
Katmai) instead of via a Xilinx generated IP core will use
this value in their device tree.

Signed-off-by: Yuri Tikhonov <yur@emcraft.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>