Allow user to set core's marchid, mvendorid, mipid CSRs through
-cpu command line option.

The default values of marchid and mipid are built with QEMU's version
numbers.

Signed-off-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Jim Shu <jim.shu@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Message-Id: <20220422040436.2233-1-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>