target/riscv: Split interrupt logic from riscv_cpu_update_mip.

This is to allow virtual interrupts to be inserted into S and VS
modes. Given virtual interrupts will be maintained in separate
mvip and hvip CSRs, riscv_cpu_update_mip will no longer be in the
path and interrupts need to be triggered for these cases from
rmw_hvip64 and rmw_mvip64 functions.

Signed-off-by: Rajnesh Kanwal <rkanwal@rivosinc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20231016111736.28721-5-rkanwal@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>