ARM: dts: dra7: add minimal l4 bus layout with control module support

This patch creates the l4_cfg and l4_wkup interconnects for DRA7, and
moves some of the generic peripherals under it. System control module
support is added to the device tree also, and the existing SCM related
functionality is moved under it.

Signed-off-by: Tero Kristo <t-kristo@ti.com>