tools: imx8m_image: align spl bin image size

tools: imx8m_image: align spl bin image size

The loader for the DDR firmware in drivers/ddr/imx/imx8m/helper.c uses a
4-byte-aligned address to load the firmware. In cases where OF is
enabled in SPL the dtb will be appended to the SPL binary and can result
in a binary that is not aligned correctly. If OF is not enabled in SPL,
`_end` is already aligned correctly, but this patch does not hurt.

To ensure the correct alignment we use dd to create a temporary file
u-boot-spl-pad.bin with the correct padding.

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>