Platform: Enable BL2 on LPC55S69

Add BL2 support and set 'BL2=ON' as default on LPC55S69.

Due to the amount of available on-chip flash memory, the following
limitations apply when BL2 is enabled:
- Build type 'Debug' cannot be selected,
- Regression tests cannot be run on the board without modifying the
  flash layout (secondary image areas must be set to 0 and in parallel
  the size of the primary regions must be increased in order for the
  S and NS images to fit in the flash).

Change-Id: I6e9a40ba202e05c86d6f91cb85c0b13674682c56
Signed-off-by: David Vincze <david.vincze@linaro.org>
diff --git a/docs/getting_started/tfm_secure_boot.rst b/docs/getting_started/tfm_secure_boot.rst
index 6be5aaf..f9f4599 100644
--- a/docs/getting_started/tfm_secure_boot.rst
+++ b/docs/getting_started/tfm_secure_boot.rst
@@ -234,9 +234,9 @@
 +---------------------+-----------------+---------------+----------+----------------+--------------+
 | AN539               | Yes             | Yes           | Yes      | Yes            | No           |
 +---------------------+-----------------+---------------+----------+----------------+--------------+
-| FVP_SSE300_MPS2     | NO              | Yes           | Yes      | Yes            | No           |
+| FVP_SSE300_MPS2     | No              | Yes           | Yes      | Yes            | No           |
 +---------------------+-----------------+---------------+----------+----------------+--------------+
-| LPC55S69            | No              | No            | No       | No             | No           |
+| LPC55S69            | Yes             | Yes           | No       | Yes            | No           |
 +---------------------+-----------------+---------------+----------+----------------+--------------+
 | Musca-A             | No              | No            | No       | No             | Yes          |
 +---------------------+-----------------+---------------+----------+----------------+--------------+