hw/riscv: acpi: only create RHCT MMU entry for supported types

hw/riscv: acpi: only create RHCT MMU entry for supported types

Do not create the RHCT MMU type entry for RV32 CPUs, since it
only has definitions for SV39/SV48/SV57.  Likewise, check that
satp_mode_max_from_map() will actually return a valid value, skipping
the MMU type entry if all MMU types were disabled on the command line.

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