dts: riscv: rename PolarFire SoC using device family name

Microchip's PolarFire SoC (device family name: MPFS) is not specific to
the Icicle Kit. Rename the devicetree sources to be more generic and to
align with Linux and allow for other development boards to adopt.

Signed-off-by: Conor Paxton <conor.paxton@microchip.com>