[U-Boot] [RFC] enforce CONFIG_DM=y

[Heinrich Schuchardt]

The time lines set force in doc/driver-model/migration.rst for different
subsystems to move to the driver model have been passed.

I ran a test on Travis CI
https://travis-ci.org/xypron2/u-boot/builds/569776600
that showed only 8 boards remain that cannot be built with CONFIG_DM=y.

H2200
* h2200

Renesas
* sh7757lcr
* sh7752evb
* r2dplus
* sh7753evb
* sh7763rdp
* MigoR
* r7780mp.

I think it is time that we enforce CONFIG_DM=y in U-Boot and remove
boards that do not support it.

Best regards

Heinrich