target/riscv: cpu: store max SATP mode as a single integer

target/riscv: cpu: store max SATP mode as a single integer

The maximum available SATP mode implies all the shorter virtual address sizes.
Store it in RISCVCPUConfig and avoid recomputing it via satp_mode_max_from_map.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>