Documentation: DT: document PRCM compatible strings for dm81x SoCs

These PRCM nodes were earlier added in patch 7800064b ("ARM: dts: Add
basic dm816x device tree configuration"), but the documentation for
the same wasn't added. Fix this by adding the missing compatible strings
under the generic prcm.txt document.
Signed-off-by: Tero Kristo <t-kristo@ti.com>

Documentation/devicetree/bindings/arm/omap/prcm.txt

@@ -29,6 +29,10 @@ Required properties:
 		"ti,dra7-prm"
 		"ti,dra7-cm-core-aon"
 		"ti,dra7-cm-core"
+		"ti,dm814-prcm"
+		"ti,dm814-scrm"
+		"ti,dm816-prcm"
+		"ti,dm816-scrm"
 - reg:		Contains PRCM module register address range
 		(base address and length)
 - clocks:	clocks for this module