[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: NPeter Bergner <bergner@vnet.ibm.com>
Signed-off-by: NPaul Mackerras <paulus@samba.org>

arch/powerpc/kernel/cputable.c

@@ -949,6 +949,7 @@ struct cpu_spec	cpu_specs[] = {
 			PPC_FEATURE_HAS_MMU | PPC_FEATURE_HAS_4xxMAC,
 		.icache_bsize		= 32,
 		.dcache_bsize		= 32,
+		.platform		= "ppc405",
 	},
 	{	/* 405EP */
 		.pvr_mask		= 0xffff0000,