ARM: dts: Renesas RZN1D-DB Board base file

This adds a base device tree file for the RZN1-DB board, with only the
basic support allowing the system to boot to a prompt. Only one UART is
used, with only a single CPU running.

Signed-off-by: Michel Pollet <michel.pollet@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>