Comment 0 for bug 1937991

[Impact]
This test will require /dev/rtc0, the default RTC device, or one
specified by user to run. Since this default RTC is not guaranteed to
exist on all of the devices, so check its existence first, otherwise
skip this test with the kselftest skip code 4.

Without this patch this test will fail like this on a s390x zVM:
$ selftests: timers: rtcpie
$ /dev/rtc0: No such file or directory
not ok 1 selftests: timers: rtcpie # exit=22

We have this test disabled in our test suite with an if check [1],
with this patch applied we can remove that code block.

[1] https://kernel.ubuntu.com/git/ubuntu/autotest-client-tests.git/tree/ubuntu_kernel_selftests/ubuntu_kernel_selftests.py#n133

[Fix]
* 0d3e5a057992bd selftests: timers: rtcpie: skip test if default RTC device does not exist

[Test]
Run this test with patched source tree on a s390x zVM and it will be
skipped correctly:
$ selftests: timers: rtcpie
$ Default RTC /dev/rtc0 does not exist. Test Skipped!
not ok 9 selftests: timers: rtcpie # SKIP

[Where problems could occur]
If this fix is incorrect we might see it failing again on systems that without /dev/rtc0 (after removing our local hacks in autotest-client-tests)