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>