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/plat/arm/board/fvp/fdts/fvp_tb_fw_config.dts b/plat/arm/board/fvp/fdts/fvp_tb_fw_config.dts
index 28299f6..ce58938 100644
--- a/plat/arm/board/fvp/fdts/fvp_tb_fw_config.dts
+++ b/plat/arm/board/fvp/fdts/fvp_tb_fw_config.dts
@@ -26,5 +26,16 @@
 		tos_fw_config_max_size = <0x200>;
 		nt_fw_config_addr = <0x0 0x80000000>;
 		nt_fw_config_max_size = <0x200>;
+		/*
+		 * The following two entries are placeholders for Mbed TLS
+		 * heap information. The default values don't matter since
+		 * they will be overwritten by BL1.
+		 * In case of having shared Mbed TLS heap between BL1 and BL2,
+		 * BL1 will populate these two properties with the respective
+		 * info about the shared heap. This info will be available for
+		 * BL2 in order to locate and re-use the heap.
+		 */
+		mbedtls_heap_addr = <0x0 0x0>;
+		mbedtls_heap_size = <0x0>;
 	};
 };