spapr: Move RMA memory region registration code

PPC970 does not support VRMA (virtual RMA) so real memory required
for SLOF to execute must be allocated by the KVM_ALLOCATE_RMA ioctl.
Later this memory is used as a part of the guest RAM area.
The RMA allocating code also registers a memory region for this piece
of RAM.

We are going to simplify memory regions layout: RMA memory region
will be a subregion in the RAM memory region, both starting from zero.
This way we will not have to take care of start address alignment for
the piece of RAM next to the RMA.

This moves memory region business closer to the RAM memory region
creation/allocation code.

As this is a mechanical patch, no change in behaviour is expected.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
[agraf: fix compilation on non-kvm systems]
Signed-off-by: Alexander Graf <agraf@suse.de>