target/riscv: Convert MIP CSR to target_ulong

target/riscv: Convert MIP CSR to target_ulong

The MIP CSR is a xlen CSR, it was only 32-bits to allow atomic access.
Now that we don't use atomics for MIP we can change this back to a xlen
CSR.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Palmer Dabbelt <palmerdabbelt@google.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>