boards: up_squared: override CPU devicetree node to have 2 CPUs

The device tree file for up_squared includes the base dts file
for Apollo Lake which only defines 1 CPU. UP Squared have
different SKUs with different CPUs, and overall has a minimal
of 2 CPUs. So amend the up_squared device tree overlay to have
2 CPU nodes.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>