Adding selftest testdata dynamically into live tree

This patch attaches selftest's device tree data (required by /drivers/of/selftest.c)
dynamically into live device tree. First, it links selftest device tree data into the
kernel image and then iterates over all the nodes and attaches them into the live tree.
Once the testcases are complete, it removes the data attached.

This patch will remove the manual process of addition and removal of selftest device
tree data into the machine's dts file.

Tested successfully with current selftest's testcases.

Signed-off-by: Gaurav Minocha <gaurav.minocha.os@gmail.com>
[glikely: Removed ability to build as a module and fixed no-devicetree bug]
Signed-off-by: Grant Likely <grant.likely@linaro.org>