core: fdtaddr: use map_sysmem() as cast for the return

For the devfdt_get_addr_index_ptr() and devfdt_get_addr_size_index_ptr()
function use map_sysmem() function as cast for the return for use in
sandbox. Also fix sandbox test.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Drop second hunk:
Signed-off-by: Simon Glass <sjg@chromium.org>