tests/drivers/rtc: Add qemu_x86_64 board to test suite

This commit adds overlay and conf for the qemu_x86_64 board
to the RTC API test suite, and adds support for the RTC
subsystem to the qemu_x86_64's yaml file.

The commit also specifies integration platforms for the
RTC API test suite, since the qemu_x86_64 board runs in
real-time, causing it to time out if the test suite runs
for it.

Signed-off-by: Bjarki Arge Andreasen <bjarkix123@gmail.com>