dm: core: Fix devfdt_get_addr_ptr return value

According to the description of devfdt_get_addr_ptr, this function should
return NULL on failure, but currently it returns (void *)FDT_ADDR_T_NONE.

Fix this by making devfdt_get_addr_ptr return NULL on failure, as
described in the function comments. Also, update the drivers currently
checking (void *)FDT_ADDR_T_NONE to check for NULL.

Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Reviewed-by: Simon Glass <sjg@chromium.org>