[POWERPC] Add AT_PLATFORM value for Xilinx Virtex-4 FX

Jakub noticed the cputable.c entry for Xilinx Virtex-4 FX was missing
a .platform value, so the AT_PLATFORM value wouldn't be set correctly.
This adds it.

Signed-off-by: Peter Bergner <bergner@vnet.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>