target/riscv: Add zimop extension

target/riscv: Add zimop extension

Zimop extension defines an encoding space for 40 MOPs.The Zimop
extension defines 32 MOP instructions named MOP.R.n, where n is
an integer between 0 and 31, inclusive. The Zimop extension
additionally defines 8 MOP instructions named MOP.RR.n, where n
is an integer between 0 and 7.

These 40 MOPs initially are defined to simply write zero to x[rd],
but are designed to be redefined by later extensions to perform some
other action.

Signed-off-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Deepak Gupta <debug@rivosinc.com>
Message-ID: <20240709113652.1239-2-zhiwei_liu@linux.alibaba.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>