Constrain interruptible key generation to key-pairs

Erroring on a symmetric-key type would actually be an extra line of
code.

In theory we could try to save that one line of code, but it is
premature optimisation at this point. Also, this is a predominantly
asymmetric crypto feature, it is less confusing/more user friendly if we
don't allow symmetric keys here.

Signed-off-by: Janos Follath <janos.follath@arm.com>
diff --git a/tf-psa-crypto/include/psa/crypto.h b/tf-psa-crypto/include/psa/crypto.h
index aabf0f7..7fd4872 100644
--- a/tf-psa-crypto/include/psa/crypto.h
+++ b/tf-psa-crypto/include/psa/crypto.h
@@ -5280,8 +5280,7 @@
  * \param[in] attributes        The attributes for the new key.
  *                              The following attributes are required for all
  *                              keys:
- *                              * The key type. It must not be an asymmetric
- *                                public key.
+ *                              * The key type. It must be an asymmetric key-pair.
  *                              * The key size. It must be a valid size for the
  *                                key type.
  *                              The following attributes must be set for keys