s390x: add vmalloc support

To use virtual addresses, we have to
- build page tables with identity mapping
- setup the primary ASCE in cr1
- enable DAT in the PSW

Not using the Linux definitions/implementation as they contain too much
software defined stuff / things we don't need.

Written from scratch. Tried to stick to the general Linux naming
schemes.

As we currently don't invalidate anything except page table entries, it
is sufficient to only use ipte for now.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20180213162321.20522-9-david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

vm