dt-bindings: Update omap l4 binding for optional registers

The interconnects on omap variants have configuration registers.
There are registers for a link agent, one or more interconnect
agent, and an optional access protection.

Let's also update the example for omap4 l4 per instance as it
has multiple interconnect agent registers.

For more information, see chapter "Interconnect" in TI techical
reference manuals.

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Tero Kristo <t-kristo@ti.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>