Mbed TLS: Remove weak heap implementation

The implementation of the heap function plat_get_mbedtls_heap() becomes
mandatory for platforms supporting TRUSTED_BOARD_BOOT.

The shared Mbed TLS heap default weak function implementation is
converted to a helper function get_mbedtls_heap_helper() which can be
used by the platforms for their own function implementation.

Change-Id: Ic8f2994e25e3d9fcd371a21ac459fdcafe07433e
Signed-off-by: Ambroise Vincent <ambroise.vincent@arm.com>
diff --git a/plat/qemu/qemu_trusted_boot.c b/plat/qemu/qemu_trusted_boot.c
index 17666b9..1ef7e43 100644
--- a/plat/qemu/qemu_trusted_boot.c
+++ b/plat/qemu/qemu_trusted_boot.c
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2017, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2017-2019, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -29,3 +29,8 @@
 {
 	return 1;
 }
+
+int plat_get_mbedtls_heap(void **heap_addr, size_t *heap_size)
+{
+	return get_mbedtls_heap_helper(heap_addr, heap_size);
+}