ARM: dts: AM33xx: Update and move wkup_m3 node to l4 node

The WakeupM3 remote processor device node has been moved to
be a child node of the newly created l4_wkup node, to reflect
its presence properly within the SoC. The node was added
previously before any driver support, it is now updated as
per the wkup_m3_rproc bindings added alongside the driver
support.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>