dt-bindings: Amlogic: Add Meson8 and Meson8b SMP related documentation

With this patch we add documentation for:

* power-management-unit: the PMU is used to bring up the cores during
  SMP operations
* sram: among other things the sram is used to store the first code
  executed by the core when it is powered up
* cpu-enable-method: the CPU enable method used by Amlogic Meson8 and
  Meson8b SoCs
Signed-off-by: Carlo Caione <carlo@endlessm.com>
[also add Meson8 to the documentation]
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>

Documentation/devicetree/bindings/arm/amlogic/pmu.txt

+Amlogic Meson8 and Meson8b power-management-unit:
+-------------------------------------------------
+
+The pmu is used to turn off and on different power domains of the SoCs
+This includes the power to the CPU cores.
+
+Required node properties:
+- compatible value : depending on the SoC this should be one of:
+			"amlogic,meson8-pmu"
+			"amlogic,meson8b-pmu"
+- reg : physical base address and the size of the registers window
+
+Example:
+
+	pmu@c81000e4 {
+		compatible = "amlogic,meson8b-pmu", "syscon";
+		reg = <0xc81000e0 0x18>;
+	};

Documentation/devicetree/bindings/arm/amlogic/smp-sram.txt

+Amlogic Meson8 and Meson8b SRAM for smp bringup:
+------------------------------------------------
+
+Amlogic's SMP-capable SoCs use part of the sram for the bringup of the cores.
+Once the core gets powered up it executes the code that is residing at a
+specific location.
+
+Therefore a reserved section sub-node has to be added to the mmio-sram
+declaration.
+
+Required sub-node properties:
+- compatible : depending on the SoC this should be one of:
+		"amlogic,meson8-smp-sram"
+		"amlogic,meson8b-smp-sram"
+
+The rest of the properties should follow the generic mmio-sram discription
+found in ../../misc/sram.txt
+
+Example:
+
+	sram: sram@d9000000 {
+		compatible = "mmio-sram";
+		reg = <0xd9000000 0x20000>;
+		#address-cells = <1>;
+		#size-cells = <1>;
+		ranges = <0 0xd9000000 0x20000>;
+
+		smp-sram@1ff80 {
+			compatible = "amlogic,meson8b-smp-sram";
+			reg = <0x1ff80 0x8>;
+		};
+	};

Documentation/devicetree/bindings/arm/cpus.txt

@@ -197,6 +197,8 @@ described below.
 			    "actions,s500-smp"
 			    "allwinner,sun6i-a31"
 			    "allwinner,sun8i-a23"
+			    "amlogic,meson8-smp"
+			    "amlogic,meson8b-smp"
 			    "arm,realview-smp"
 			    "brcm,bcm11351-cpu-method"
 			    "brcm,bcm23550"