hw/core: fix platform bus node name

hw/core: fix platform bus node name

"platform" is not a valid name for a bus node in dt-schema, so warnings
can be see in dt-validate on a dump of the riscv virt dtb:

/stuff/qemu/qemu.dtb: platform@4000000: $nodename:0: 'platform@4000000' does not match '^([a-z][a-z0-9\\-]+-bus|bus|soc|axi|ahb|apb)(@[0-9a-f]+)?$'
        From schema: /home/conor/.local/lib/python3.9/site-packages/dtschema/schemas/simple-bus.yaml
"platform-bus" is a valid name, so use that instead.

CC: Rob Herring <robh@kernel.org>
Fixes: 11d306b9df ("hw/arm/sysbus-fdt: helpers for platform bus nodes addition")
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Message-id: 20220810184612.157317-5-mail@conchuod.ie
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>