target/riscv: Allow setting CPU feature from machine/device emulation

target/riscv: Allow setting CPU feature from machine/device emulation

The machine or device emulation should be able to force set certain
CPU features because:
1) We can have certain CPU features which are in-general optional
   but implemented by RISC-V CPUs on the machine.
2) We can have devices which require a certain CPU feature. For example,
   AIA IMSIC devices expect AIA CSRs implemented by RISC-V CPUs.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Signed-off-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Frank Chang <frank.chang@sifive.com>
Message-id: 20220204174700.534953-6-anup@brainfault.org
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>