powerpc/powernv: Increase candidate fw image size

At present we assume candidate image is <= 256MB. But in P8,
candidate image size can go up to 750MB. Hence increasing
candidate image max size to 1GB.
Signed-off-by: NVasant Hegde <hegdevasant@linux.vnet.ibm.com>
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/platforms/powernv/opal-flash.c

@@ -76,8 +76,8 @@
 /* Validate buffer size */
 #define VALIDATE_BUF_SIZE	4096
 
-/* XXX: Assume candidate image size is <= 256MB */
-#define MAX_IMAGE_SIZE	0x10000000
+/* XXX: Assume candidate image size is <= 1GB */
+#define MAX_IMAGE_SIZE	0x40000000
 
 /* Flash sg list version */
 #define SG_LIST_VERSION (1UL)