ARM: shmobile: porter: initial device tree

Add the initial device tree for the R8A7791 SoC based Porter low cost board
(which  is a  slightly modified version  of the Henninger board).

SCIF0 serial port support is included, so that the serial console can work.

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