spapr: Enable dynamic change of the supported hypercalls list

spapr: Enable dynamic change of the supported hypercalls list

At the moment the "ibm,hypertas-functions" list is fixed. However some
calls should be listed there if they are supported by QEMU or the host
kernel.

This enables hyperrtas_prop to grow on stack by adding
a SPAPR_HYPERRTAS_ADD macro. "qemu,hypertas-functions" is converted as well.

The first user of this is going to be a "multi-tce" property.

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