feat(fvp): enable external SP images in BL2 config

Currently the list of SP UUIDs loaded by BL2 is hardcoded in the DT.
This is a problem when building a system with other SPs (e.g. from
Trusted Services). This commit implements a workaround to enable adding
SP UUIDs to the list at build time.

Signed-off-by: Balint Dobszay <balint.dobszay@arm.com>
Change-Id: Iff85d3778596d23d777dec458f131bd7a8647031
diff --git a/docs/plat/arm/arm-build-options.rst b/docs/plat/arm/arm-build-options.rst
index d4fa98d..74251bd 100644
--- a/docs/plat/arm/arm-build-options.rst
+++ b/docs/plat/arm/arm-build-options.rst
@@ -100,6 +100,9 @@
 -  ``ARM_SPMC_MANIFEST_DTS`` : path to an alternate manifest file used as the
    SPMC Core manifest. Valid when ``SPD=spmd`` is selected.
 
+-  ``ARM_BL2_SP_LIST_DTS``: Path to DTS file snippet to override the hardcoded
+   SP nodes in tb_fw_config.
+
 -  ``OPTEE_SP_FW_CONFIG``: DTC build flag to include OP-TEE as SP in tb_fw_config
    device tree. This flag is defined only when ``ARM_SPMC_MANIFEST_DTS`` manifest
    file name contains pattern optee_sp.