dm: core: fix devfdt_remap_addr_index()

commit 30a90f56 ("dm: core: add functions to get memory-mapped I/O
addresses") introduced a devfdt_remap_addr_index() routine but it does
not make use of the index parameter.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>