uart: export rclk via sysctl

To help debugging, export the rclk a uart is using as
dev.uart.X.rclk. It can be opaque when it is wrong since any error
messages printed to the system console using the wrong rclk aren't
informative.

Sponsored by: Netflix
Reviewed by: andrew, markj
Differential Revision: https://reviews.freebsd.org/D47070