target/riscv: update max_satp_mode based on QOM properties

target/riscv: update max_satp_mode based on QOM properties

Almost all users of cpu->cfg.satp_mode care about the "max" value
satp_mode_max_from_map(cpu->cfg.satp_mode.map).  Convert the QOM
properties back into it.  For TCG, deduce the bitmap of supported modes
from valid_vm[].

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