arm: dts: imx8m: update binman ddr firmware node name

We are migrating to use binman symbols, the current names are
inconsistent across different boards, so unify them.

Also add `type = "blob-ext";`, since the new names are not valid binman
types.

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: Edit commit message]
Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>