fdt: Update fdt_get_base_address() to use const

This function does not change the device tree so adjust it to use const
for this parameter.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/common/fdt_support.c b/common/fdt_support.c
index c6a76b7..c63b27b 100644
--- a/common/fdt_support.c
+++ b/common/fdt_support.c
@@ -1539,7 +1539,7 @@
 /*
  * Returns the base address of an SOC or PCI node
  */
-u64 fdt_get_base_address(void *fdt, int node)
+u64 fdt_get_base_address(const void *fdt, int node)
 {
 	int size;
 	u32 naddr;