arm: am33xx: Handle NAND+I2C boot-device the same way as NAND

Re-map NAND&I2C boot-device to the "normal" NAND boot-device.
Otherwise the SPL boot IF can't handle this device correctly.
Somehow booting with Hynix 4GBit NAND H27U4G8 on Siemens
Draco leads to this boot-device passed to SPL from the BootROM.

With this change, Draco boots just fine into main U-Boot.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Tom Rini <trini@ti.com>
Cc: Roger Meier <r.meier@siemens.com>
Cc: Samuel Egli <samuel.egli@siemens.com>