ARM: socfpga: dts: document "altr,stratix10-rst-mgr" binding

"altr,stratix10-rst-mgr" is used for the Stratix10 reset manager.

Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>