Attest: Replace crypto related size definitions

Replace hard-coded values with the PSA Crypto macros
to calculate the size of the ECC public key.

Change-Id: I613e10d67eb968bd47a3f40c014b743003c9a9ed
Signed-off-by: Tamas Ban <tamas.ban@arm.com>
diff --git a/platform/include/tfm_plat_crypto_keys.h b/platform/include/tfm_plat_crypto_keys.h
index d9a21b3..82202f0 100644
--- a/platform/include/tfm_plat_crypto_keys.h
+++ b/platform/include/tfm_plat_crypto_keys.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2017-2019, Arm Limited. All rights reserved.
+ * Copyright (c) 2017-2020, Arm Limited. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  *
@@ -62,8 +62,6 @@
     uint32_t  puby_key_size;
 };
 
-#define ECC_P_256_KEY_SIZE  (96u)  /* 3 x 32 = 96 bytes priv + pub-x + pub-y */
-
 #define ROTPK_HASH_LEN (32u) /* SHA256 */
 
 /**