package/freescale-imx: Add option for DDR FW need

package/freescale-imx: Add option for DDR FW need

Only some i.MX8 needs a DDR training firmware (8M, 8MM, 8MN). Some other
i.MX8 (QuadMax, QuadXPlus) rely on system controller for that task.

Suggested-by: Julien Olivain <julien.olivain@oss.nxp.com>
Signed-off-by: Stephane Viau <stephane.viau@oss.nxp.com>
Reviewed-by: Gary Bisson <gary.bisson@boundarydevices.com>
[yann.morin.1998@free.fr:
  - use the new variable to drive the firmware installation
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>