riscv/virt: Manually define the machine

riscv/virt: Manually define the machine

Instead of using the DEFINE_MACHINE() macro to define the machine let's
do it manually. This allows us to use the machine object to create
RISCVVirtState. This is required to add children and aliases to the
machine.

This patch is no functional change.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>