proposal: crypto/ecdh: support for P224

[koh-osug]

The new added package is missing support for P-224 curves. This gives a 112 bit security and is still good enough comparable to RSA 2048 bit keys used in the field. The P-224 curve plays a role for resource constrained devices like smart cards.

[seankhliao]

cc @golang/security

[robinwesterik]

I'm still working with the "deprecated" crypto/elliptic and crypto/curve packages because of this. As Karsten mentioned, a lot of embedded devices rely on P-224 curves for security