ear: update Nonce size limits in validate()

Update Nonce size limits to align with the latest updates to the EAT
draft:

ietf-rats-wg/eat#421

This also helps ups deal with CCA which mandates realm challenge to be
64 bytes that would exceed the old size limit when base64 encoded.

Signed-off-by: Sergei Trofimov <sergei.trofimov@arm.com>

ear.go

@@ -154,7 +154,7 @@ func (o AttestationResult) validate() error {
 
 	if o.Nonce != nil {
 		nLen := len(*o.Nonce)
-		if nLen > 74 || nLen < 10 {
+		if nLen > 88 || nLen < 8 {
 			invalid = append(invalid, fmt.Sprintf("eat_nonce (%d bytes)", nLen))
 		}
 	}