feat(handoff): add 32-bit variant of SRAM layout

Introduce the 32-bit variant of the SRAM layout used by BL1 to
communicate available free SRAM to BL2. This layout was added to the
specification in:
https://github.com/FirmwareHandoff/firmware_handoff/pull/54.

Change-Id: I559fb8a00725eaedf01856af42d73029802aa095
Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>
diff --git a/include/lib/transfer_list.h b/include/lib/transfer_list.h
index c864526..bdc6349 100644
--- a/include/lib/transfer_list.h
+++ b/include/lib/transfer_list.h
@@ -63,6 +63,7 @@
 	TL_TAG_SRAM_LAYOUT64 = 0x104,
 	TL_TAG_MBEDTLS_HEAP_INFO = 0x105,
 	TL_TAG_EXEC_EP_INFO32 = 0x106,
+	TL_TAG_SRAM_LAYOUT32 = 0x107,
 };
 
 enum transfer_list_ops {