of: unittest: Create overlay_common.dtsi and testcases_common.dtsi

In order to build-test the same unit-test files using fdtoverlay tool,
move the device nodes from the existing overlay_base.dts and
testcases_common.dts files to .dtsi counterparts. The .dts files now
include the new .dtsi files, resulting in exactly the same behavior as
earlier.

The .dtsi files can now be reused for compile time tests using
fdtoverlay (will be done by a later commit).

This is required because the base files passed to fdtoverlay tool
shouldn't be overlays themselves (i.e. shouldn't have the /plugin/;
tag).

Note that this commit also moves "testcase-device2" node to
testcases.dts from tests-interrupts.dtsi, as this node has a deliberate
error in it and is only relevant for runtime testing done with
unittest.c.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Frank Rowand <frank.rowand@sony.com>
Tested-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/c3354a042ba34a03fd563061cbaa7fc96cb2d71a.1615354376.git.viresh.kumar@linaro.org