sh: r2dplus: Enable DM

Enable driver model support for SH4 R2Dplus board. Thus far, no
drivers are bound via the DM. The PCI drivers have yet to be
converted to DM_PCI.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>