fdt: Fix coverity complaint about 32-bit multiplication

Coverity raised an eyebrow over our GICR frame size calculation:
========
    CID 362942:  Integer handling issues  (OVERFLOW_BEFORE_WIDEN)
Potentially overflowing expression "nr_cores * gicr_frame_size" with type
"unsigned int" (32 bits, unsigned) is evaluated using 32-bit arithmetic,
and then used in a context that expects an expression of type "uint64_t"
(64 bits, unsigned).
========

Even with a GICv4 (256KB frame size) we need 16384 cores to overflow
32-bit, so it's not a practical issue.

But it's also easy to fix, so let's just do that: cast gicr_frame_size
to an unsigned 64-bit integer, so that the multiplication is done in the
64-bit realm.

Change-Id: Iad10e19b9e58d5fbf9d13205fbcef0aac5ae48af
Signed-off-by: Andre Przywara <andre.przywara@arm.com>

common/fdt_fixup.c

@@ -425,7 +425,8 @@ int fdt_adjust_gic_redist(void *dtb, unsigned int nr_cores,
 		redist_size_32 = cpu_to_fdt32(nr_cores * gicr_frame_size);
 		val = &redist_size_32;
 	} else {
-		redist_size_64 = cpu_to_fdt64(nr_cores * gicr_frame_size);
+		redist_size_64 = cpu_to_fdt64(nr_cores *
+					      (uint64_t)gicr_frame_size);
 		val = &redist_size_64;
 	}