Support shared Mbed TLS heap for FVP

This patch introduces the shared Mbed TLS heap optimisation for Arm
platforms. The objective is the Mbed TLS heap to be shared between BL1
and BL2 so as to not allocate the heap memory twice. To achieve that,
the patch introduces all the necessary helpers for implementing this
optimisation. It also applies it for FVP.

Change-Id: I6d85eaa1361517b7490956b2ac50f5fa0d0bb008
Signed-off-by: John Tsichritzis <john.tsichritzis@arm.com>
diff --git a/include/plat/arm/common/plat_arm.h b/include/plat/arm/common/plat_arm.h
index 1af4dd1..0770c0e 100644
--- a/include/plat/arm/common/plat_arm.h
+++ b/include/plat/arm/common/plat_arm.h
@@ -223,6 +223,8 @@
 void arm_load_tb_fw_config(void);
 void arm_bl2_set_tb_cfg_addr(void *dtb);
 void arm_bl2_dyn_cfg_init(void);
+void arm_bl1_set_mbedtls_heap(void);
+int arm_get_mbedtls_heap(void **heap_addr, size_t *heap_size);
 
 /*
  * Mandatory functions required in ARM standard platforms