dm: core: Add DM_FLAG_OS_PREPARE flag

This new flag can be added to DM device drivers, which need to do some
final configuration before U-Boot exits and the OS (e.g. Linux) is
started. The remove functions of those drivers will get called at
this stage to do these last-stage configuration steps.

Signed-off-by: Stefan Roese <sr@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Cc: Bin Meng <bmeng.cn@gmail.com>