uboot-imx

mx6cuboxi: Use more standard namings for fdt variables

README file suggests to use 'fdtfile' for the dtb file name and
'fdt_addr_r' for the dtb address in RAM, so do as suggested.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Reviewed-by: Tom Rini <trini@konsulko.com>