hw/riscv: virt: fix DT property mmu-type when CPU mmu option is disabled

hw/riscv: virt: fix DT property mmu-type when CPU mmu option is disabled

The device tree property "mmu-type" is currently exported as either
"riscv,sv32" or "riscv,sv48".

However, the riscv cpu device tree binding [1] has a specific value
"riscv,none" for a HART without a MMU.

Set the device tree property "mmu-type" to "riscv,none" when the CPU mmu
option is disabled using rv32,mmu=off or rv64,mmu=off.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/devicetree/bindings/riscv/cpus.yaml?h=v5.17

Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20220414155510.1364147-1-niklas.cassel@wdc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>