linux-phytec-fsl

Documentation: Update kselftest.txt

Add document for how to install selftests.

Signed-off-by: Wang Long <long.wanglong@huawei.com>
Acked-by: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>