spapr: nested: Document Nested PAPR API

Adding initial documentation about Nested PAPR API to describe the set
of APIs and its usage. Also talks about the Guest State Buffer elements
and it's format which is used between L0/L1 to communicate L2 state.

Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Harsh Prateek Bora <harshpb@linux.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
1 file changed