ppc/xive2: Change context/ring specific functions to be generic

ppc/xive2: Change context/ring specific functions to be generic

Some the functions that have been created are specific to a ring or context. Some
of these same functions are being changed to operate on any ring/context. This  will
simplify the next patch sets that are adding additional ring/context operations.

Signed-off-by: Michael Kowal <kowal@linux.ibm.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>