soc: arm: add Makefiles one level up

It was not possible to add directories under an SoC family
because of the direct call to Makefile in the SoC series from the
architecture level.

While we have SoC still using the old structure, add a conditional
to still support old structure and allow Makefiles directly under
an SoC family directory. 

This is useful for adding drivers and BSP files common to one family.

Change-Id: I85dc8341523b41949de91d78675153ce0baa5aac
Signed-off-by: Anas Nashif <anas.nashif@intel.com>