riscv: dts: microchip: move sysctrlr out of soc bus

The MPFS system controller has no registers of its own, so move it out
of the soc node to avoid dtbs_check warnings:
arch/riscv/boot/dts/microchip/microchip-mpfs-icicle-kit.dtb: soc: syscontroller: {'compatible': ['microchip,mpfs-sys-controller'], 'mboxes': [[15, 0]], 'status': ['okay']} should not be valid under {'type': 'object'}

Reported-by: Palmer Dabbelt <palmer@rivosinc.com>
Suggested-by: Rob Herring <robh@kernel.org>
Fixes: 528a5b1f

 ("riscv: dts: microchip: add new peripherals to icicle kit device tree")
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20220509142610.128590-3-conor.dooley@microchip.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>