ARM: dts: rskrza1: initial device tree

Add the initial device tree for the RZ/A1 based development board (RSK).

Signed-off-by: Chris Brandt <chris.brandt@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>