spapr: Render full FDT on ibm,client-architecture-support

The ibm,client-architecture-support call is a way for the guest to
negotiate capabilities with a hypervisor. It is implemented as:
- the guest calls SLOF via client interface;
- SLOF calls QEMU (H_CAS hypercall) with an options vector from the guest;
- QEMU returns a device tree diff (which uses FDT format with
an additional header before it);
- SLOF walks through the partial diff tree and updates its internal tree
with the values from the diff.

This changes QEMU to simply re-render the entire tree and send it as
an update. SLOF can handle this already mostly, [1] is needed before this
can be applied. This stores the resulting tree in the spapr machine to have
the latest valid FDT copy possible (this should not matter much as
H_UPDATE_DT happens right after that but nevertheless).

The benefit is reduced code size as there is no need for another set of
DT rendering helpers such as spapr_fixup_cpu_dt().

The downside is that the updates are bigger now (as they include all
nodes and properties) but the difference on a '-smp 256,threads=1' system
before/after is 2.35s vs. 2.5s.

[1] https://patchwork.ozlabs.org/patch/1152915/

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

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