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>

Documentation/devicetree/bindings/reset/socfpga-reset.txt

 Altera SOCFPGA Reset Manager
 
 Required properties:
-- compatible : "altr,rst-mgr"
+- compatible : "altr,rst-mgr" for (Cyclone5/Arria5/Arria10)
+	       "altr,stratix10-rst-mgr","altr,rst-mgr" for Stratix10 ARM64 SoC
 - reg : Should contain 1 register ranges(address and length)
 - altr,modrst-offset : Should contain the offset of the first modrst register.
 - #reset-cells: 1