[crypto] Record that an OTBN division helper function runs in constant time.

Follow-up to a code review comment on OTBN division; this note was
initially missing.

Signed-off-by: Jade Philipoom <jadep@google.com>

sw/otbn/crypto/div.s

diff --git a/sw/otbn/crypto/div.s b/sw/otbn/crypto/div.s
index 2030310..804c593 100644
--- a/sw/otbn/crypto/div.s
+++ b/sw/otbn/crypto/div.s
@@ -122,6 +122,8 @@
  *   for k=0..n-i-1:
  *     r[k+i], carry = addc(r[k+i], y[k] & mask, carry)
  *
+ * This routine runs in constant time.
+ *
  * Flags: Flags have no meaning beyond the scope of this subroutine.
  *
  * @param[in]    x8: i, number of limbs to shift y (i < n)