riscv: dts: fix the mpfs's reference clock frequency

The initial devicetree for PolarFire SoC incorrectly created a fixed
frequency clock in the devicetree to represent the msspll, but the
msspll is not a fixed frequency clock. The actual reference clock on a
board is either 125 or 100 MHz, 125 MHz in the case of the icicle kit.
Swap the incorrect representation of the msspll out for the actual
reference clock.

Fixes: dd4ee416 ("riscv: dts: Add device tree for Microchip Icicle Kit")
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>
Reviewed-by: Padmarao Begari <padmarao.begari@microchip.com>