rst_n is optional in LiteEth's PHYs.

Now possible with https://github.com/enjoy-digital/litedram/commit/2f5784432d2bd42686c91ea148fcb7849317cf5d.

platforms/fpc_iii: review/cleanup to increase similarities with others platforms and ease maintenance.

FPC-III is the Free Permutable Computer; details on the board are
available from:

    https://repo.or.cz/fpc-iii.git