debugfs: add SMC channel

Provide an SMC interface to the 9p filesystem. This permits
accessing firmware drivers through a common interface, using
standardized read/write/control operations.

Signed-off-by: Ambroise Vincent <ambroise.vincent@arm.com>
Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
Change-Id: I9314662314bb060f6bc02714476574da158b2a7d
diff --git a/docs/getting_started/build-options.rst b/docs/getting_started/build-options.rst
index 051586b..469fd34 100644
--- a/docs/getting_started/build-options.rst
+++ b/docs/getting_started/build-options.rst
@@ -544,6 +544,10 @@
    (Coherent memory region is included) or 0 (Coherent memory region is
    excluded). Default is 1.
 
+-  ``USE_DEBUGFS``: When set to 1 this option activates an EXPERIMENTAL feature
+   exposing a virtual filesystem interface through BL31 as a SiP SMC function.
+   Default is 0.
+
 -  ``USE_ROMLIB``: This flag determines whether library at ROM will be used.
    This feature creates a library of functions to be placed in ROM and thus
    reduces SRAM usage. Refer to :ref:`Library at ROM` for further details. Default