docs(spm): document new build option

Add documentation for the new build option ENABLE_SPMD_LP.

Signed-off-by: Raghu Krishnamurthy <raghu.ncstate@gmail.com>
Change-Id: I808e6c00e3699fc900dc97e889af63cc01cae794
diff --git a/docs/getting_started/build-options.rst b/docs/getting_started/build-options.rst
index c410a8e..c665715 100644
--- a/docs/getting_started/build-options.rst
+++ b/docs/getting_started/build-options.rst
@@ -899,6 +899,12 @@
    support pre-Armv8.4 platforms (aka not implementing the ``FEAT_SEL2``
    extension).
 
+-  ``ENABLE_SPMD_LP`` : This boolean option is used jointly with the SPM
+   Dispatcher option (``SPD=spmd``). When enabled (1) it indicates support
+   for logical partitions in EL3, managed by the SPMD as defined in the FF-A
+   1.2 specification. This flag is disabled by default. This flag must not be
+   used if ``SPMC_AT_EL3`` is enabled. This is an experimental feature.
+
 -  ``SPM_MM`` : Boolean option to enable the Management Mode (MM)-based Secure
    Partition Manager (SPM) implementation. The default value is ``0``
    (disabled). This option cannot be enabled (``1``) when SPM Dispatcher is