core: fdtaddr: add devfdt_get_addr_size_index_ptr function

Add devfdt_get_addr_size_index_ptr function with the same
functionality as devfdt_get_addr_size_index, but instead
a return pointer is given.

Suggested-by: Michael Nazzareno Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Reviewed-by: Michael Trimarchi <michael@amarulasolutions.com>
Reviewed-by: Simon Glass <sjg@chromium.org>