target/riscv: Fix typo of mimpid cpu option

target/riscv: Fix typo of mimpid cpu option

"mimpid" cpu option was mistyped to "mipid".

Fixes: 9951ba94 ("target/riscv: Support configuarable marchid, mvendorid, mipid CSR values")
Signed-off-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20220523153147.15371-1-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>