The timebase-frequency on PolarFire SoC is not set by an oscillator on
the board, but rather by an internal divider, so move the property to
mpfs.dtsi.

This looks to be copy-pasta from the SiFive Unleashed as the comments
in both places were almost identical. In the Unleashed's case this looks
to actually be valid, as the clock is provided by a crystal on the PCB.
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
---
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