arm: dts: imx8m: shrink ddr firmware size to actual file size

After we switch to use BINMAN_SYMBOLS, there is no need to pad
the file size to 0x8000 and 0x4000. After we use BINMAN_SYMBOLS,
the u-boot-spl-ddr.bin shrink about 36KB with i.MX8MP-EVK.

Tested-by: Tim Harvey <tharvey@gateworks.com> #imx8m[m,n,p]-venice
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>