memory: Add TI GPMC driver

The GPMC is a unified memory controller dedicated for interfacing
with external memory devices like
 - Asynchronous SRAM-like memories and ASICs
 - Asynchronous, synchronous, and page mode burst NOR flash
 - NAND flash
 - Pseudo-SRAM devices

This driver will take care of setting up the GPMC based on
the settings specified in the Device tree and then
probe its children.

Signed-off-by: Roger Quadros <rogerq@kernel.org>