dt-bindings: sram: convert rockchip-pmu-sram bindings to yaml

Current dts files with 'rockchip-pmu-sram' compatible nodes
are now verified with sram.yaml, although the original
text document still exists. Merge rockchip-pmu-sram.txt
with sram.yaml by adding it as description with an example.
Make #address-cells, #size-cells and ranges optional
if there are no child nodes to prevent yaml warnings.

Signed-off-by: Johan Jonker <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>

Documentation/devicetree/bindings/sram/rockchip-pmu-sram.txt

···
-Rockchip SRAM for pmu:
-------------------------------
-
-The sram of pmu is used to store the function of resume from maskrom(the 1st
-level loader). This is a common use of the "pmu-sram" because it keeps power
-even in low power states in the system.
-
-Required node properties:
-- compatible : should be "rockchip,rk3288-pmu-sram"
-- reg : physical base address and the size of the registers window
-
-Example:
-	sram@ff720000 {
-		compatible = "rockchip,rk3288-pmu-sram", "mmio-sram";
-		reg = <0xff720000 0x1000>;
-	};

Documentation/devicetree/bindings/sram/sram.yaml

···
       enum:
         - mmio-sram
         - atmel,sama5d2-securam
+        - rockchip,rk3288-pmu-sram
 
   reg:
     maxItems: 1
···
 required:
   - compatible
   - reg
-  - "#address-cells"
-  - "#size-cells"
-  - ranges
+
+if:
+  properties:
+    compatible:
+      contains:
+        const: rockchip,rk3288-pmu-sram
+
+else:
+  required:
+    - "#address-cells"
+    - "#size-cells"
+    - ranges
 
 additionalProperties: false
 
···
             compatible = "rockchip,rk3066-smp-sram";
             reg = <0x10080000 0x50>;
         };
+    };
+
+  - |
+    // Rockchip's rk3288 SoC uses the sram of pmu to store the function of
+    // resume from maskrom(the 1st level loader). This is a common use of
+    // the "pmu-sram" because it keeps power even in low power states
+    // in the system.
+    sram@ff720000 {
+      compatible = "rockchip,rk3288-pmu-sram", "mmio-sram";
+      reg = <0xff720000 0x1000>;
     };
 
   - |