docs(bl31): aarch64: RESET_TO_BL31_WITH_PARAMS

Document the RESET_TO_BL31 with parameters feature.

Signed-off-by: Jorge Ramirez-Ortiz <jorge@foundries.io>
Change-Id: I47014d724f2eb822b69a112c3acee546fbfe82d5
diff --git a/docs/getting_started/build-options.rst b/docs/getting_started/build-options.rst
index 4dbf5cb..46d5740 100644
--- a/docs/getting_started/build-options.rst
+++ b/docs/getting_started/build-options.rst
@@ -709,6 +709,11 @@
    entrypoint) or 1 (CPU reset to BL31 entrypoint).
    The default value is 0.
 
+-  ``RESET_TO_BL31_WITH_PARAMS``: If ``RESET_TO_BL31`` has been enabled, setting
+   this additional option guarantees that the input registers are not cleared
+   therefore allowing parameters to be passed to the BL31 entrypoint.
+   The default value is 0.
+
 -  ``RESET_TO_SP_MIN``: SP_MIN is the minimal AArch32 Secure Payload provided
    in TF-A. This flag configures SP_MIN entrypoint as the CPU reset vector
    instead of the BL1 entrypoint. It can take the value 0 (CPU reset to BL1