Documentation: update the of_selftest.txt

Since the directory "drivers/of/testcase-data" is renamed
to "drivers/of/unittest-data". so we should update the path
in the of_selftest.txt.

When the kernel is built with OF_SELFUNIT enabled, the output
dtb is testcases.dtb instead of testcase.dtb, also update it
(s/testcase/testcases/).

Signed-off-by: Wang Long <long.wanglong@huawei.com>
Signed-off-by: Rob Herring <robh@kernel.org>