arm64: dts: imx8mm-u-boot.dtsi: improve odd blob-ext naming

Rather than using odd implicit blob-ext naming, explicitly specify the
type to be of blob-ext and therefore also simplify the node naming.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>