spapr: increase temporary fdt buffer size

At the moment the size of the buffer is set to 64K which is
enough for approximately 150 VCPUs which is not the limit.

This increases the buffer up to 256K which allows having
a tree for approximately 600 VCPUs which is way beyond the real
number we need.

As only the real size of the tree is copied to the guest, there
will be no impact on existing configurations.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Alexander Graf <agraf@suse.de>
