uart: Implement SPCR rev 3 and 4 for PreciseBaudrate and UartClkFreq

If we have a new enough SPCR, then use it when it provides a
PreciseBaudrate and/or a UartClkFreq.

Sponsored by: Netflix
Reviewed by: andrew,adrian
Differential Revision: https://reviews.freebsd.org/D47097