dm: core: Add a new API devfdt_get_addr_index_ptr()

At present there is only devfdt_get_addr_ptr() which only returns
the first <addr, size> pair in the 'reg' property. Add a new API
devfdt_get_addr_index_ptr() to return the indexed pointer.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>