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>