virtio-rtc: Add device specification

[quic-philber]

The RTC (Real Time Clock) device provides information about current
time. The device can provide different clocks, for the UTC or TAI time
standards, or for physical time elapsed since some past epoch. For UTC
clocks, the device can also indicate how leap seconds are handled. The
driver can read the clocks with simple or more accurate methods.
Optionally, the driver can set an alarm.

The driver-side timekeeping can synchronize precisely to a local clock
exposed by the RTC device. If supported by the device, the driver can
obtain a cross-timestamp from the local clock and the system hardware
counter, in a way similar to the Linux kernel ptp_kvm driver.

The alarm allows the driver side to be woken up from a sleep state at a
specific time.

Latest proposal: https://lore.kernel.org/virtio-comment/[email protected]/

Linux kernel driver compliant to latest proposal: https://lore.kernel.org/lkml/[email protected]/

[quic-philber]

(Updated the issue to refer to spec v9 instead of v8.)