Switch to an SPL image. The SPL for Ventana does the following:
 - setup i2c and read the factory programmed EEPROM to obtain DRAM config
   and model for board-specific calibration data
 - configure DRAM per CPU/size/layout/devices/calibration
 - load u-boot.img from NAND and jump to it

This allows for a single SPL+u-boot.img to replace the previous multiple boa
configurations.

Cc: Stefan Roese <sr@denx.de>
Cc: Otavio Salvador <otavio@ossystems.com.br>
Cc: Andy Ng <andreas2025@gmail.com>
Cc: Eric Nelson <eric.nelson@boundarydevices.com>
Cc: Tapani Utriainen <tapani@technexion.com>
Cc: Tom Rini <trini@ti.com>

Signed-off-by: Tim Harvey <tharvey@gateworks.com>