Add driver model functionality for generic board.

This includes data structures and base code for registering devices and
uclasses (groups of devices with the same purpose, e.g. all I2C ports will
be in the same uclass).

The feature is enabled with CONFIG_DM.
Signed-off-by: NSimon Glass <sjg@chromium.org>
Signed-off-by: NMarek Vasut <marex@denx.de>
Signed-off-by: NPavel Herrmann <morpheus.ibis@gmail.com>
Signed-off-by: NViktor Křivák <viktor.krivak@gmail.com>
Signed-off-by: NTomas Hlavacek <tmshlvck@gmail.com>