dt-bindings: omap: add new binding for PRM instances

Add new binding for OMAP PRM (Power and Reset Manager) instances. Each
of these will act as a power domain controller and potentially as a reset
provider.
Signed-off-by: Tero Kristo <t-kristo@ti.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>

Documentation/devicetree/bindings/arm/omap/prm-inst.txt

+OMAP PRM instance bindings
+
+Power and Reset Manager is an IP block on OMAP family of devices which
+handle the power domains and their current state, and provide reset
+handling for the domains and/or separate IP blocks under the power domain
+hierarchy.
+
+Required properties:
+- compatible:	Must contain one of the following:
+		"ti,am3-prm-inst"
+		"ti,am4-prm-inst"
+		"ti,omap4-prm-inst"
+		"ti,omap5-prm-inst"
+		"ti,dra7-prm-inst"
+		and additionally must contain:
+		"ti,omap-prm-inst"
+- reg:		Contains PRM instance register address range
+		(base address and length)
+
+Optional properties:
+- #reset-cells:	Should be 1 if the PRM instance in question supports resets.
+
+Example:
+
+prm_dsp2: prm@1b00 {
+	compatible = "ti,dra7-prm-inst", "ti,omap-prm-inst";
+	reg = <0x1b00 0x40>;
+	#reset-cells = <1>;
+};