ARM: dts: dra7: add PRM nodes

Add PRM nodes for dra7 series of SoCs. These are initially used to
support reset control for some of the nodes, but will be extended
later to add powerdomain control and support for PRCM irqs among
other things.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>