selftests: timers: clocksource-switch: add 'runtime' command line parameter

So the user can decide how long the test should run.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: John Stultz <jstultz@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>