Remove unneeded include paths in PLAT_INCLUDES

Also, update platform_def.h guidelines about includes in the porting
guide.

Change-Id: I1ae338c9dd3242b309f6d53687ba2cb755d488c3
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>
diff --git a/docs/porting-guide.rst b/docs/porting-guide.rst
index 7fc5297..30a863d 100644
--- a/docs/porting-guide.rst
+++ b/docs/porting-guide.rst
@@ -103,20 +103,13 @@
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Each platform must ensure that a header file of this name is in the system
-include path with the following constants defined. This may require updating the
-list of ``PLAT_INCLUDES`` in the ``platform.mk`` file. In the Arm development
-platforms, this file is found in ``plat/arm/board/<plat_name>/include/``.
+include path with the following constants defined. This will require updating
+the list of ``PLAT_INCLUDES`` in the ``platform.mk`` file.
 
 Platform ports may optionally use the file `include/plat/common/common\_def.h`_,
 which provides typical values for some of the constants below. These values are
 likely to be suitable for all platform ports.
 
-Platform ports that want to be aligned with standard Arm platforms (for example
-FVP and Juno) may also use `include/plat/arm/common/arm\_def.h`_, which provides
-standard values for some of the constants below. However, this requires the
-platform port to define additional platform porting constants in
-``platform_def.h``. These additional constants are not documented here.
-
 -  **#define : PLATFORM\_LINKER\_FORMAT**
 
    Defines the linker format used by the platform, for example