board/freescale/common/imx: use generic ddr_fw.bin name

board/freescale/common/imx: use generic ddr_fw.bin name

For some SoCs (i.e.: i.MX8M Nano for now), the DDR training firmware
in the boot image depends on the type of memory installed on a board
(LPDDR4, DDR4, etc...). The previous patch makes this firmware name
agnostic of the DDR type and uses ddr_fw.bin as a generic name. This
patch makes imx8-bootloader-prepare.sh use this generic name,
independently of the kind board DDR type.

Signed-off-by: Stephane Viau <stephane.viau@oss.nxp.com>
Reviewed-by: Maeva Manuel <maeva.manuel@oss.nxp.com>
Reviewed-by: Julien Olivain <julien.olivain@oss.nxp.com>
Reviewed-by: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>