hw/riscv: virt: fix the plic's address cells

hw/riscv: virt: fix the plic's address cells

When optional AIA PLIC support was added the to the virt machine, the
address cells property was removed leading the issues with dt-validate
on a dump from the virt machine:
/stuff/qemu/qemu.dtb: plic@c000000: '#address-cells' is a required property
        From schema: /stuff/linux/Documentation/devicetree/bindings/interrupt-controller/sifive,plic-1.0.0.yaml
Add back the property to suppress the warning.

Reported-by: Rob Herring <robh@kernel.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Message-id: 20220810184612.157317-3-mail@conchuod.ie
Link: https://lore.kernel.org/linux-riscv/20220803170552.GA2250266-robh@kernel.org/
Fixes: e6faee6585 ("hw/riscv: virt: Add optional AIA APLIC support to virt machine")
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>