uart: Prefer rclk passed in over rclk in the class

If rclk is set in sysdev, then it was set during the boot process and is
intended to override the defaults. By prefering the sysdev one over the
class, xo=XXXX in hw.uart.console can give the user a usable console for
non-traditional UARTs, especially on !x86 platforms. The default rclk
generally only is good for I/O mapped UARTS or PCI ones that we can do a
table lookup on. Other times, it can be hard to know what a good default
is without more information.

Sponsored by: Netflix
Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D47069