core: read: add dev_read_addr_index_ptr function

Add dev_read_addr_index_ptr function with the
same functionality as dev_read_addr_index,
but instead a return pointer is given.
Use map_sysmem() function as cast for the return.
Make same fix for dev_read_addr_ptr() function.

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