s390x: reduce TARGET_PHYS_ADDR_SPACE_BITS to 62

With the next patch, the memory API will complain if the
TARGET_PHYS_ADDR_SPACE_BITS gets dangerously close to an
overflow.  s390x can handle up to 64 bit of physical address
space from its page tables, but we never use that much.  Just
decrease the value.

Cc: Alexander Graf <agraf@suse.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>