riscv: dts: microchip: move timebase-frequency to mpfs.dtsi

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>
---
CC: Conor Dooley <conor.dooley@microchip.com>
CC: Daire McNamara <daire.mcnamara@microchip.com>
CC: Rob Herring <robh+dt@kernel.org>
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CC: Paul Walmsley <paul.walmsley@sifive.com>
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