debugfs: add 9p device interface

The 9p interface provides abstraction layers allowing the software
that uses devices to be independent from the hardware.

This patch provides a file system abstraction to link drivers to their
devices and propose a common interface to expose driver operations to
higher layers. This file system can be used to access and configure a
device by doing read/write operations.

Signed-off-by: Ambroise Vincent <ambroise.vincent@arm.com>
Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
Change-Id: Ia9662393baf489855dc0c8f389fe4a0afbc9c255
diff --git a/make_helpers/defaults.mk b/make_helpers/defaults.mk
index 348b3e5..797f75c 100644
--- a/make_helpers/defaults.mk
+++ b/make_helpers/defaults.mk
@@ -194,6 +194,9 @@
 # Build option to choose whether Trusted Firmware uses Coherent memory or not.
 USE_COHERENT_MEM		:= 1
 
+# Build option to add debugfs support
+USE_DEBUGFS			:= 0
+
 # Build option to choose whether Trusted Firmware uses library at ROM
 USE_ROMLIB			:= 0