ARM: shmobile: add r8a7793 minimal SoC device tree

Minimal r8a7793 device tree including one CPU core, interrupt controllers,
timers, two serial ports, and the Ethernet controller, plus the required
clock descriptions.

Signed-off-by: Ulrich Hecht <ulrich.hecht+renesas@gmail.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>