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.
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