dt-bindings: altera: document syscon as fallback for sys-mgr

For 32-bit Altera SoCFPGA parts, the sys-mgr uses the syscon as a fallback.

This change addresses this warning from dtbs_check:

sysmgr@ffd08000 (altr,sys-mgr): compatible: 'oneOf' conditional failed, one
must be fixed:
	['altr,sys-mgr', 'syscon'] is too long
	'altr,sys-mgr-s10' was expected
	'altr,sys-mgr' was expected
from schema $id: http://devicetree.org/schemas/soc/altera/altr,sys-mgr.yaml

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>

Documentation/devicetree/bindings/soc/altera/altr,sys-mgr.yaml

@@ -13,7 +13,9 @@ properties:
   compatible:
     oneOf:
       - description: Cyclone5/Arria5/Arria10
-        const: altr,sys-mgr
+        items:
+          - const: altr,sys-mgr
+          - const: syscon
       - description: Stratix10 SoC
         items:
           - const: altr,sys-mgr-s10
@@ -45,7 +47,7 @@ additionalProperties: false
 examples:
   - |
     sysmgr@ffd08000 {
-      compatible = "altr,sys-mgr";
+      compatible = "altr,sys-mgr", "syscon";
       reg = <0xffd08000 0x1000>;
       cpu1-start-addr = <0xffd080c4>;
     };