ddr: imx8m: helper: load ddr firmware according to binman symbols

By reading binman symbols, we no need hard coded IMEM_LEN/DMEM_LEN after
we update the binman dtsi to drop 0x8000/0x4000 length for the firmware.

And that could save binary size for many KBs.

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>
[Alper: Check BINMAN_SYMS_OK instead]
Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>