rpc-hw-fix: Changes required for testing RPC-DRAM

[gregdavill]

RPC DRAM from Etron reduces the pin count of DRAM interfaces by offering a serial Address/Cmd interface.

It's available in both a standard 96 ball DDR3 package, and a WLCSP. The current OrangeCrab boards should be capable of supporting the 96-ball package with some hw changes.

There is a small errata with the RPC part:

The CS# pin (ball D-3, which is UDM for a DDR3) needs a 50 ohm termination resistor located near the dram and the other end connected to VDDQ
Without the resistor you may get some pretty bad signal integrity / overshoot & undershoot that may possibly cause some problems

Hardware Changes:

• Set VDD_DRAM to 1.5V:
  • R32 <= 100k
• Attach 51R termination between CS# (UDM pin on DDR3) and VDDQ
  • PN: RC0402FR-0751RL

51R suggested placement location

Note: flipped board view. (viewed from back of PCB):