imgtool: Remove P224 curve references

Imgtool does not provide support for P224
curve, just a placeholder function that
says it's not currently implemented.
This has now been removed as P224 support
had been removed from the boot code too.

Signed-off-by: Roland Mikhel <roland.mikhel@arm.com>
Change-Id: I477d8e273085f38f35eaf9a591584f3e937d748d
diff --git a/docs/encrypted_images.md b/docs/encrypted_images.md
index 239668c..834428f 100644
--- a/docs/encrypted_images.md
+++ b/docs/encrypted_images.md
@@ -148,7 +148,7 @@
 ## [Creating your keys with imgtool](#creating-your-keys-with-imgtool)
 
 `imgtool` can generate keys by using `imgtool keygen -k <output.pem> -t <type>`,
- where type can be one of `rsa-2048`, `rsa-3072`, `ecdsa-p256`, `ecdsa-p224`
+ where type can be one of `rsa-2048`, `rsa-3072`, `ecdsa-p256`
 or `ed25519`. This will generate a keypair or private key.
 
 To extract the public key in source file form, use