Documentation/soc/intel/fsp/ppi: Document new feature to dispatch external PPI

Some new feature added into FSP specification to perform dispatching
of external PPI service from boot firmware (coreboot) to FSP.

Change-Id: Iaf6b54ccd27e21860539bb2a9966054fdb027108
Signed-off-by: Subrata Banik <subrata.banik@intel.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/31839
Reviewed-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-by: Philipp Deppenwiese <zaolin.daisuki@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

Documentation/soc/intel/fsp/index.md

@@ -11,3 +11,7 @@ This section contains documentation about Intel-FSP in public domain.
 * [FSP Specification 1.1](https://www.intel.com/content/dam/www/public/us/en/documents/technical-specifications/fsp-architecture-spec-v1-1.pdf)
 
 * [FSP Specification 2.0](https://www.intel.com/content/dam/www/public/us/en/documents/technical-specifications/fsp-architecture-spec-v2.pdf)
+
+## Additional Features in FSP 2.1 specification
+
+- [PPI](ppi/ppi.md)

Documentation/soc/intel/fsp/ppi/ppi.md

@@ -0,0 +1,9 @@
+# PEIM to PEIM Interface (PPI)
+
+This section is intended to document the purpose of creating PPI service
+inside coreboot space to perform some specific operation related to CPU,
+chipset using Intel FSP. This feature is added into FSP specification 2.1
+where FSP should be able to locate PPI, published by boot firmware and
+able to execute the same in FSP's context.
+
+* [What is PPI](https://www.intel.com/content/dam/www/public/us/en/documents/reference-guides/efi-pei-cis-v09.pdf)