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

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