ppc: spapr: Introduce FWNMI capability

ppc: spapr: Introduce FWNMI capability

Introduce fwnmi an spapr capability and add a helper function
which tries to enable it, which would be used by following patch
of the series. This patch by itself does not change the existing
behavior.

Signed-off-by: Aravinda Prasad <arawinda.p@gmail.com>
[eliminate cap_ppc_fwnmi, add fwnmi cap to migration state
 and reprhase the commit message]
Signed-off-by: Ganesh Goudar <ganeshgr@linux.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Message-Id: <20200130184423.20519-3-ganeshgr@linux.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>