ARM: dts: add Plymovent BAS board

Plymovent BAS is a base system controller produced for the Plymovent filter
systems.

Co-Developed-by: David Jander <david@protonic.nl>
Signed-off-by: David Jander <david@protonic.nl>
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>