prci: register tlclk as a fixed clock

The PRCI exports tlclk as a constant fixed divisor clock, defined as 1/2
of the coreclk frequency. In older FU540 device trees (such as the one
provided by SiFive), tlclk is represented as its own entity, and is
automatically registered as a fixed-divisor-clock. Unfortunately the
upstream FU540 device tree (that we have in our tree) represents tlclk
as an output of the PRCI block, and we must register it manually. At
worst, users of the old device tree will end up with an unreferenced
duplicate of tlclk.

This fixes device attachment for the SiFive UART on newer device trees,
since it references tlclk via the PRCI.

Reviewed by: kp
Differential Revision: https://reviews.freebsd.org/D23406