powerpc/powernv: Infrastructure to support OPAL async completion

This patch adds support for notifying the clients of their request
completion. Clients request for the token before making OPAL call
and then wait for the response.

This patch uses messaging infrastructure to pull the data to linux
by registering itself for the message type OPAL_MSG_ASYNC_COMP.

Signed-off-by: Neelesh Gupta <neelegup@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>