tests/drivers/rtc: Add MC146818 driver to test suite

This commit updates the qemu_x86 board's yaml file to
indicate its support for the RTC subsystem. Board overlay
and conf for the qemu_x86 has been added to the RTC test
suite to enable the MC146818 and its dependencies.

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