dm: Fix error handling for dev_read_addr_ptr

dm: Fix error handling for dev_read_addr_ptr

dev_read_addr_ptr had different semantics depending on whether OF_LIVE was
enabled. This patch converts both implementations to return NULL on error,
and converts all call sites which check for FDT_ADDR_T_NONE to check for
NULL instead. This patch also removes the call to map_physmem, since we
have dev_remap_addr* for those semantics.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>