doc: board: renesas: Document Renesas RZ/N1D and RZ/N1S as arm

The RZ/N1D and RZ/N1S contain Cortex-A7 core, which is 32bit ARM core.
Document the SoC as 32bit ARM instead of aarch64.

Fixes: a5b9f959439b ("doc: renesas: add Renesas board docs")
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>