mx53loco: Change 'fdt_file' dynamically

Since kernel 3.15 there are two dtb's for the imx53-qsb board:

imx53-qsb.dtb - For the boards with DA9053 PMIC
imx53-qsrb.dtb - For the boards with MC34708 PMIC

Change the 'fdt_file' dynamically, so that the correct dtb can be used depending
on the board variant.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>