submodules: add FSP mirror as non-default submodule

Like the 3rdparty/blobs repo this isn't checked out by default. Right
now you can manually check it out using

    $ git submodule init --checkout

A follow up commit will add some automagic if USE_BLOBS and
MAINBOARD_USES_FSP2_0 are enabled.

Change-Id: Ie612495abc2a2d5947225e6ab54872aa72d4bec6
Signed-off-by: Patrick Georgi <patrick@georgi.software>
Reviewed-on: https://review.coreboot.org/28303
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

.gitmodules

@@ -21,3 +21,8 @@
 [submodule "libgfxinit"]
 	path = 3rdparty/libgfxinit
 	url = ../libgfxinit.git
+[submodule "3rdparty/fsp"]
+	path = 3rdparty/fsp
+	url = ../fsp.git
+	update = none
+	ignore = dirty