ARM: dts: wheat: initial device tree

Add the initial device tree for the R8A7792  SoC based Wheat board.
The Wheat board itself has  no serial ports  wired up, the USB serial chips
are situated on a separate  debug board and  one of them is connected to
SCFI0  -- include unconditional support for  it, so that the serial console
can work.

Based  on the original (and large) patch by Vladimir Barinov
<vladimir.barinov@cogentembedded.com>.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>