Add private HCALL to inform updated RTAS base and entry

This patch adds a private HCALL to inform qemu the updated
rtas-base and rtas-entry address when OS invokes the call
"instantiate-rtas". This is required as qemu allocates the
error reporting structure in RTAS space upon a machine check
exception and hence needs to know the updated RTAS.

Enhancements to qemu to handle the private HCALL, prepare
error log and invoke machine check notification routine
are in a separate patch.

Signed-off-by: Nikunj A. Dadhania <nikunj@linux.vnet.ibm.com>
Signed-off-by: Aravinda Prasad <aravinda@linux.vnet.ibm.com>
Signed-off-by: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>