hw/riscv/virt.c: imsics DT: add '#msi-cells'

hw/riscv/virt.c: imsics DT: add '#msi-cells'

The DT docs for riscv,imsics [1] requires a 'msi-cell' property. Add one
and set it zero.

[1] Documentation/devicetree/bindings/interrupt-controller/riscv,imsics.yaml

Reported-by: Conor Dooley <conor@kernel.org>
Fixes: 28d8c281200f ("hw/riscv: virt: Add optional AIA IMSIC support to virt machine")
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-ID: <20240531202759.911601-9-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>