Adding the open-source-pdks repository under `third_party`.

Goal is to include common information about open source PDKs into the
GF180MCU PDK documentation. This includes;
 * File naming information -
   https://github.com/google/open-source-pdks/issues/7
 * Versioning scheme -
   https://github.com/google/open-source-pdks/issues/8
 * Other open source PDKs that exist.

Signed-off-by: Tim 'mithro' Ansell <tansell@google.com>

.gitmodules

diff --git a/.gitmodules b/.gitmodules
index 3f12985..49bea27 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -39,3 +39,6 @@
 	url = https://github.com/google/globalfoundries-pdk-ip-gf180mcu_fd_ip_sram.git
 	branch = main
 	shallow = true
+[submodule "third_party/open-source-pdks"]
+	path = third_party/open-source-pdks
+	url = https://github.com/google/open-source-pdks.git

third_party/open-source-pdks

diff --git a/third_party/open-source-pdks b/third_party/open-source-pdks
new file mode 160000
index 0000000..fd509b9
--- /dev/null
+++ b/third_party/open-source-pdks
@@ -0,0 +1 @@
+Subproject commit fd509b924cb902dd0650d04889a95072040144ee