riscv bhyve: add peripheral clock

[br]

Summary

Add a peripheral clock node and use it for UART.
This is needed for Linux.

[jrtc27]

It doesn't need clocks, it needs one of clocks or clock-frequency. QEMU's virt board lists the latter, which is far simpler and preferable in a virtualisation setting. It also uses 3686400 as the frequency, which I believe lets the guest go up to 230400 baud. We should align with QEMU where possible in order to avoid surprises.

[br]

Add clock-frequency property to UART instead of defining fixed-clock