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>
diff --git a/drivers/of/of_reserved_mem.c b/drivers/of/of_reserved_mem.c
index daaaf93..e420eb5 100644
--- a/drivers/of/of_reserved_mem.c
+++ b/drivers/of/of_reserved_mem.c
@@ -95,8 +95,8 @@
 	int t_len = (dt_root_addr_cells + dt_root_size_cells) * sizeof(__be32);
 	phys_addr_t start = 0, end = 0;
 	phys_addr_t base = 0, align = 0, size;
-	unsigned long len;
-	__be32 *prop;
+	int len;
+	const __be32 *prop;
 	int nomap;
 	int ret;