To remove non-ascii characters in of_selftest.txt

This patch removes the non-ascii characters in
Documentation/devicetree/of_selftest.txt

Signed-off-by: Gaurav Minocha <gaurav.minocha.os@gmail.com>
[grant.likely: Fix: s/of_fdt_unflatten_device_tree()/of_fdt_unflatten_tree()/]
Signed-off-by: Grant Likely <grant.likely@linaro.org>