juno/cdcel: Make output Y1 optional

This patch adds a compile-time option such that the
handling of the output Y1 can be removed from the
implementation. This does not affect functionality
for Juno and is primarily done for memory footprint
improvement.

Change-Id: Id816ead046ccb93abee558c0f632c62bf24a8040
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>