uart/ns8250: Add support for computing rclk

When instructed to do so, compute the rclk (baud rate generator clock)
based on the currently programmed divisor and the communicated baud
rate. We only do this once and only for consoles that tell us the
configured rate and flag we can likely safely compute rclk.

Sponsored by: Netflix
Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D47076