dt-bindings: soc/microchip: update syscontroller compatibles

The Polarfire SoC is currently using two different compatible string
prefixes. Fix this by changing "polarfire-soc-*" strings to "mpfs-*" in
its system controller in order to match the compatible string used in
the soc binding and device tree

Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>