mem: Add tWR to DRAM activate and precharge constraints

mem: Add tWR to DRAM activate and precharge constraints

This patch adds the write recovery time to the DRAM timing
constraints, and changes the current tRASDoneAt to a more generic
preAllowedAt, capturing when a precharge is allowed to take place.

The part of the DRAM access code that accounts for the precharge and
activate constraints is updated accordingly.