of: Fixing OF Selftest build error

This patch is to fix following error while compiling OF selftests.
"drivers/of/selftest.c:617:2: error: implicit declaration of function
'of_fdt_unflatten_tree'"

Now, CONFIG_OF_SELFTEST depends on CONFIG_OF_EARLY_FLATTREE so that the
broken configuration cannot be selected. Ultimately it would be a good
idea to allow CONFIG_OF_SELFTEST to select CONFIG_OF_EARLY_FLATTREE, but
there is a dependency problem on i386 and x86_64 that causes dtc to not
get built and causes the build to fail. That problem needs to be fixed
first.

Signed-off-by: Gaurav Minocha <gaurav.minocha.os@gmail.com>
Signed-off-by: Grant Likely <grant.likely@linaro.org>