Use DT_MACHINE_START() on the r8a7740 based armadillo 800 eva 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>