hw/ppc/spapr: Add some missing hcall function set strings

Add "hcall-sprg0" (for H_SET_SPRG0), "hcall-copy" (for H_PAGE_INIT)
and "hcall-debug" (for H_LOGICAL_CI_LOAD/STORE) to the property
"ibm,hypertas-functions" to indicate that we support these hypercalls.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>