Add the standard cells as submodules.

They are being added as submodules from the
https://foss-eda-tools.googlesource.com/ repository due to their large
size. People are encouraged to only initialize and clone the submodules
needed for their designs.

 * [`sky130_fd_sc_hvl`](https://github.com/google/skywater-pdk/tree/master/libraries/sky130_fd_sc_hvl) -
   "High voltage" digital standard cells provided by the SkyWater
   foundry.

 * [`sky130_fd_sc_lp`](https://github.com/google/skywater-pdk/tree/master/libraries/sky130_fd_sc_hp) -
   "Low power" digital standard cells provided by the SkyWater foundry.

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