of/unittest: Show broken behaviour in the platform bus

Add a single resource to the test bus device to exercise the platform
bus code a little more. This isn't strictly a devicetree test, but it is
a corner case that the devicetree runs into. Until we've got platform
device unittests, it can live here. It doesn't need to be an explicit
text because the kernel will oops when it is wrong.

Cc: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Cc: Rob Herring <robh@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Grant Likely <grant.likely@linaro.org>
[wsa: added the comment provided by Grant, rebased, and tested]
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Rob Herring <robh@kernel.org>