Add Ethernet and PLIC

[Yuan-Mao]

This PR adds Ethernet and PLIC.

[dpetrisko]

Hi, we want to be careful about integrating external code. Rearrange like so:

ethernet_controller/v/ ; New files
ethernet_controller/test/ ; I guess makefile to run test, if the actual TB is integrated in ethernet_controller code. If it's a new tb it can go here
ethernet_controller/import/ ; vendor files
ethernet_controller/patch.txt ; git patch for modification to vendor files
ethernet_controller/README ; commit id and description of changes to vendor code
ethernet_controller/LICENSE ; LICENSE of vendor files, from git repo

[Yuan-Mao]

Sounds good, but how would the patch.txt be useful? Isn't it enough to just include the LICENSE?

[dpetrisko]

patch is useful to show quickly what the diff is. Otherwise it's up to the user to clone the upstream repo and do a manual diff

[dpetrisko]

See https://github.com/lowRISC/opentitan/tree/master/hw/vendor/patches/pulp_riscv_dbg for an example

[Yuan-Mao]

Might need to pull out the reset for iodelay ctrl as well.
Also need to consider the sim for ssio_ddr_in

[taylor-bsg]

@Yuan-Mao @dpetrisko what remains to finalize this pull request?