mem: Add precharge all (PREA) to the DRAM controller

mem: Add precharge all (PREA) to the DRAM controller

This patch adds the basic ingredients for a precharge all operation,
to be used in conjunction with DRAM power modelling.

Currently we do not try and apply any cleverness when precharging all
banks, thus even if only a single bank is open we use PREA as opposed
to PRE. At the moment we only have a single tRP (tRPpb), and do not
model the slightly longer all-bank precharge constraint (tRPab).