dm: core: Correct bad cast in ofnode_get_addr_size_index()

At present this code passes an fdt_addr_t pointer as a u64 pointer which
is not safe, since sizeof(fdt_addr_t) may be 4, e.g. with sandbox. Correct
this to avoid a stack corruption problem.

Fixes: e679d03b (core: ofnode: Add ofnode_get_addr_size_index)
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
[bmeng: correct one typo in the commit message]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>