feat(el3-spmc): add a flag to enable support to load SEL0 SP

Introduce a build flag for enabling the support for loading SEL0 SP in
EL3 SPMC.

Signed-off-by: Nishant Sharma <nishant.sharma@arm.com>
Change-Id: I1d63ae4d0d8374a732113565be90d58861506e39

docs/components/secure-partition-manager.rst

@@ -152,6 +152,8 @@ SPMC located at S-EL1, S-EL2 or EL3:
   exception level is set to S-EL1.
   ``SPMD_SPM_AT_SEL2`` is enabled. The context save/restore routine
   and exhaustive list of registers is visible at `[4]`_.
+- **SPMC_AT_EL3_SEL0_SP**: this option enables the support to load SEL0 SP
+  when SPMC at EL3 support is enabled.
 - **SP_LAYOUT_FILE**: this option specifies a text description file
   providing paths to SP binary images and manifests in DTS format
   (see `Describing secure partitions`_). It
@@ -257,6 +259,22 @@ Sample TF-A build command line when the SPMC is located at EL3:
     PLAT=fvp \
     all fip
 
+Sample TF-A build command line when the SPMC is located at EL3 and SEL0 SP is
+enabled:
+
+.. code:: shell
+
+    make \
+    CROSS_COMPILE=aarch64-none-elf- \
+    SPD=spmd \
+    SPMD_SPM_AT_SEL2=0 \
+    SPMC_AT_EL3=1 \
+    SPMC_AT_EL3_SEL0_SP=1 \
+    BL32=<path-to-tee-binary> \
+    BL33=<path-to-bl33-binary> \
+    PLAT=fvp \
+    all fip
+
 FVP model invocation
 ====================