configs: add defconfig for Freescale i.MX31 PDK

The i.MX31 Product Development Kit (or PDK) is Freescale development board
based on the i.MX31 application processor.

This defconfig is based on mainline kernel v3.15.10, and is aimed at a PDK in
"3 stack" configuration, with CPU engine board, personality board and debug
board.

Signed-off-by: Vincent Stehlé <vincent.stehle@freescale.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>