fix(cpus): update doc and check for plat_can_cmo

plat_can_cmo must not clobber x1 but the doc doesn't mention that. This
patch updates the doc to mention x1. It also adds check for plat_can_cmo
to `dcsw_op_louis` which was missed out in original patch.

Signed-off-by: Okash Khawaja <okash@google.com>
Change-Id: I721376bf3726520d0d5b0df0f33f98ce92257287