hw/riscv: virt: fix uart node name

hw/riscv: virt: fix uart node name

"uart" is not a node name that complies with the dt-schema.
Change the node name to "serial" to ix warnings seen during
dt-validate on a dtbdump of the virt machine such as:
/stuff/qemu/qemu.dtb: uart@10000000: $nodename:0: 'uart@10000000' does not match '^serial(@.*)?$'
        From schema: /stuff/linux/Documentation/devicetree/bindings/serial/8250.yaml

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: 20220810184612.157317-2-mail@conchuod.ie
Link: https://lore.kernel.org/linux-riscv/20220803170552.GA2250266-robh@kernel.org/
Fixes: 04331d0b56 ("RISC-V VirtIO Machine")
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>