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

docs/getting_started/porting-guide.rst

@@ -1502,8 +1502,8 @@ When CONDITIONAL_CMO flag is enabled:
 - This function indicates whether cache management operations should be
   performed. It returns 0 if CMOs should be skipped and non-zero
   otherwise.
-- The function must not clobber x2 and x3. It's also not safe to rely on stack.
-  Otherwise obey AAPCS.
+- The function must not clobber x1, x2 and x3. It's also not safe to rely on
+  stack. Otherwise obey AAPCS.
 
 Modifications specific to a Boot Loader stage
 ---------------------------------------------

lib/aarch64/cache_helpers.S

@@ -217,6 +217,7 @@ dcsw_loop_table:
 
 
 func dcsw_op_louis
+	check_plat_can_cmo
 	dcsw_op #LOUIS_SHIFT, #CLIDR_FIELD_WIDTH, #LEVEL_SHIFT
 endfunc dcsw_op_louis