pseries: Consolidate construction of /rtas device tree node

pseries: Consolidate construction of /rtas device tree node

For historical reasons construction of the /rtas node in the device
tree (amongst others) is split into several places.  In particular
it's split between spapr_create_fdt_skel(), spapr_build_fdt() and
spapr_rtas_device_tree_setup().

In fact, as well as adding the actual RTAS tokens to the device tree,
spapr_rtas_device_tree_setup() just adds the ibm,lrdr-capacity
property, which despite going in the /rtas node, doesn't have a lot to
do with RTAS.

This patch consolidates the code constructing /rtas together into a new
spapr_dt_rtas() function.  spapr_rtas_device_tree_setup() is renamed to
spapr_dt_rtas_tokens() and now only adds the token properties.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>