of/fdt: update of_get_flat_dt_prop in prep for libfdt

Make of_get_flat_dt_prop arguments compatible with libfdt fdt_getprop
call in preparation to convert FDT code to use libfdt. Make the return
value const and the property length ptr type an int.

Signed-off-by: Rob Herring <robh@kernel.org>
Tested-by: Michal Simek <michal.simek@xilinx.com>
Tested-by: Grant Likely <grant.likely@linaro.org>
Tested-by: Stephen Chivers <schivers@csc.com>

arch/arc/kernel/devtree.c

@@ -42,7 +42,7 @@ const struct machine_desc * __init setup_machine_fdt(void *dt)
 	const struct machine_desc *mdesc;
 	unsigned long dt_root;
 	void *clk;
-	unsigned long len;
+	int len;
 
 	if (!early_init_dt_scan(dt))
 		return NULL;