ppc: spapr-rtas - implement os-term rtas call

PAPR compliant guest calls this in absence of kdump. This finally
reaches the guest and can be handled according to the policies set by
higher level tools(like taking dump) for further analysis by tools like
crash.

Linux kernel calls ibm,os-term when extended property of os-term is set.
This makes sure that a return to the linux kernel is gauranteed.

Signed-off-by: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>
[agraf: reduce RTAS_TOKEN_MAX]
Signed-off-by: Alexander Graf <agraf@suse.de>