ARM: dts: Move am4 l3 noc to a separate node

In preparation for probing l3 with simple-pm-bus and genpd, we must move
l3 noc to a separate node to prevent omap_l3_noc.c driver from claiming
the whole l3 instance before simple-pm-bus has a chance to probe.

Signed-off-by: Tony Lindgren <tony@atomide.com>