docs/system: riscv: Update description of CPU

docs/system: riscv: Update description of CPU

Since the hypervisor extension been non experimental and enabled for
default CPU, the previous command is no longer available and the
option `x-h=true` or `h=true` is also no longer required.

Signed-off-by: Yu Li <liyu.yukiteru@bytedance.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <9040401e-8f87-ef4a-d840-6703f08d068c@bytedance.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>