selftests: rtc: skip when RTC is not present

There is no point in failing the tests when RTC is not present.

Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Tested-by: Daniel Diaz <daniel.diaz@linaro.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>