dt-bindings: arm: zte: Move sysctrl bindings to their own doc

In preparation to convert board-level bindings to json-schema, move
various misc SoC bindings out to their own file.

Cc: Mark Rutland <[e-mail]>
Cc: Jun Nie <[e-mail]>
Cc: Baoyou Xie <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Acked-by: Shawn Guo <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>

Documentation/devicetree/bindings/arm/zte,sysctrl.txt

···
+ZTE sysctrl Registers
+
+Registers for 'zte,zx296702' SoC:
+
+System management required properties:
+      - compatible = "zte,sysctrl"
+
+Low power management required properties:
+      - compatible = "zte,zx296702-pcu"
+
+Bus matrix required properties:
+      - compatible = "zte,zx-bus-matrix"
+
+
+Registers for 'zte,zx296718' SoC:
+
+System management required properties:
+      - compatible = "zte,zx296718-aon-sysctrl"
+      - compatible = "zte,zx296718-sysctrl"
+
+Example:
+aon_sysctrl: aon-sysctrl@116000 {
+	compatible = "zte,zx296718-aon-sysctrl", "syscon";
+	reg = <0x116000 0x1000>;
+};
+
+sysctrl: sysctrl@1463000 {
+	compatible = "zte,zx296718-sysctrl", "syscon";
+	reg = <0x1463000 0x1000>;
+};

Documentation/devicetree/bindings/arm/zte.txt

···
 ZTE platforms device tree bindings
----------------------------------------
 
+---------------------------------------
 -  ZX296702 board:
     Required root node properties:
       - compatible = "zte,zx296702-ad1", "zte,zx296702"
-
-System management required properties:
-      - compatible = "zte,sysctrl"
-
-Low power management required properties:
-      - compatible = "zte,zx296702-pcu"
-
-Bus matrix required properties:
-      - compatible = "zte,zx-bus-matrix"
-
 
 ---------------------------------------
 -  ZX296718 SoC:
···
 
 ZX296718 EVB board:
       - "zte,zx296718-evb"
-
-System management required properties:
-      - compatible = "zte,zx296718-aon-sysctrl"
-      - compatible = "zte,zx296718-sysctrl"
-
-Example:
-aon_sysctrl: aon-sysctrl@116000 {
-	compatible = "zte,zx296718-aon-sysctrl", "syscon";
-	reg = <0x116000 0x1000>;
-};
-
-sysctrl: sysctrl@1463000 {
-	compatible = "zte,zx296718-sysctrl", "syscon";
-	reg = <0x1463000 0x1000>;
-};