board: schneider: add RZN1 board support

Add support for Schneider Electric RZ/N1D and RZ/N1S boards, which
are based on the Reneasas RZ/N1 SoC devices.

The intention is to support both boards using a single defconfig, and to
handle the differences at runtime.

Signed-off-by: Ralph Siemsen <ralph.siemsen@linaro.org>
Reviewed-by: Marek Vasut <marek.vasut+renesas@mailbox.org>