Some i.MX53 want to setup the SDRAM from C code rather than
from DCD tables. The image size for these boards is limited
to the internal SRAM size. To overcome this limitation for
i.MX53 boards booting from NAND implement an xload mechanism
to load only the PBL to SRAM and let barebox load the rest
of the image itself after SDRAM has been initialized.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>