Use DT_MACHINE_START() on the sh73a0 based KZM9G board.

Also include a tiny DTS file to describe the board and
update the Kconfig dependencies to select CONFIG_USE_OF.
Signed-off-by: NMagnus Damm <damm@opensource.se>
Signed-off-by: NRafael J. Wysocki <rjw@sisk.pl>