Documentation: Add sbs-manager device tree node documentation

This patch adds device tree documentation for the sbs-manager

Signed-off-by: Karl-Heinz Schneider <karl-heinz@schneider-inet.de>
Signed-off-by: Phil Reid <preid@electromag.com.au>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>