Tested with a CPU frequency of 70MHz and a nest frequency of 50MHz

[RCS Arctic tern] Add alternate uses for LPC header signals

Sparrowhawk is a single CPU POWER9 OpenVPX platform
Arctic Tern is a ModBMC-based platform for BMC development

The Kestrel development kit consists of a Versa ECP5-45 board
along with two adapter PCBs and cabling required to connect
the board to a Blackbird or Talos II host system.

This reduces FPGA bitstream load time from Flash well below
1 second.

Thanks to mithro for the pointer to the ECP5 bistream packer
options!

This frees up additional space for new peripherals in the future.

Rework peripheral address map to fit new device
Make debug pads optional

The LiteX timer peripheral wastes resources and is not required
for operation of the BIOS on POWER SoCs.

Initial refactor of kestrel code - will be used for versa adapter board changes.

See merge request !7

This also explicitly sets the CSR stride to avoid breakage
under future LiteX upstream changes.

This change does not alter the generated HDL or bitstream.

Successfully tested with all Kestrel peripherals and Ethernet enabled

arrow_sockit: get video terminal working on VGA

targets/nexys_video: Revert clk100 to avoid breaking Linux-on-LiteX-VexRiscv (we'll remove it when the switch the simple framebuffer will be done).