ddr: imx8m: fix ddr firmware location when enable SPL OF

ddr: imx8m: fix ddr firmware location when enable SPL OF

With CONFIG_SPL_OF_CONTROL, the device tree will be padded to
end of the u-boot-spl-nodtb.bin, however we also put
the ddr firmware file to this location, so need to adapt
the code with SPL OF and align to 4 bytes to ease copy firmware.

Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Tested-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Peng Fan <peng.fan@nxp.com>