SE keys: save the bit size in storage

For a key in a secure element, save the bit size alongside the slot
number.

This is a quick-and-dirty implementation where the storage format
depends on sizeof(size_t), which is fragile. This should be replaced
by a more robust implementation before going into production.
diff --git a/library/psa_crypto_slot_management.c b/library/psa_crypto_slot_management.c
index 6b87ea0..e63dcda 100644
--- a/library/psa_crypto_slot_management.c
+++ b/library/psa_crypto_slot_management.c
@@ -138,13 +138,12 @@
 #if defined(MBEDTLS_PSA_CRYPTO_SE_C)
     if( psa_key_lifetime_is_external( p_slot->lifetime ) )
     {
-        if( key_data_length != sizeof( p_slot->data.se.slot_number ) )
+        if( key_data_length != sizeof( p_slot->data.se ) )
         {
             status = PSA_ERROR_STORAGE_FAILURE;
             goto exit;
         }
-        memcpy( &p_slot->data.se.slot_number, key_data,
-                sizeof( p_slot->data.se.slot_number ) );
+        memcpy( &p_slot->data.se, key_data, sizeof( p_slot->data.se ) );
     }
     else
 #endif /* MBEDTLS_PSA_CRYPTO_SE_C */