selftest: rtc: Check if could access /dev/rtc0 before testing

The rtctest requires the read permission on /dev/rtc0. The rtctest will
be skipped if the /dev/rtc0 is not readable.

Reviewed-by: Koba Ko <kobak@nvidia.com>
Reviewed-by: Matthew R. Ochs <mochs@nvidia.com>
Signed-off-by: Joseph Jang <jjang@nvidia.com>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>