Add generic DT board support for the r8a7740 SoC.

SCIF serial ports and timers are kept as regular
platform devices. Other on-chip and on-board devices
should be configured via the device tree.

At this point there is no interrupt controller support
in place but such code will be added over time when
proper IRQ domain support has been added to INTC.
Signed-off-by: NMagnus Damm <damm@opensource.se>
Signed-off-by: NRafael J. Wysocki <rjw@sisk.pl>