riscv: dts: microchip: mpfs: Fix reference clock node

"make dtbs_check" reports:

    arch/riscv/boot/dts/microchip/microchip-mpfs-icicle-kit.dt.yaml: soc: refclk: {'compatible': ['fixed-clock'], '#clock-cells': [[0]], 'clock-frequency': [[600000000]], 'clock-output-names': ['msspllclk'], 'phandle': [[7]]} should not be valid under {'type': 'object'}
	From schema: dtschema/schemas/simple-bus.yaml

Fix this by moving the node out of the "soc" subnode.
While at it, rename it to "msspllclk", and drop the now superfluous
"clock-output-names" property.
Move the actual clock-frequency value to the board DTS, since it is not
set until bitstream programming time.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Tested-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>