package/linux-firmware: remove imx sdma firmware

package/linux-firmware: remove imx sdma firmware

As reported on the mailing list [1], the imx sdma firmwares for
imx[6,7,8] are provided by two packages: firmware-imx and
linux-firmware

$ sha256sum ./firmware-imx-8.10/firmware/sdma/sdma-imx6q.bin
7790c161b7e013a9dbcbffb17cc5d4cb63d952949a505647e4679f02d04c4784
./firmware-imx-8.10/firmware/sdma/sdma-imx6q.bin

$ sha256sum  ./linux-firmware-20201022/imx/sdma/sdma-imx6q.bin
7790c161b7e013a9dbcbffb17cc5d4cb63d952949a505647e4679f02d04c4784
./linux-firmware-20201022/imx/sdma/sdma-imx6q.bin

firmware-imx also has firmwares for older variants as well, so this is
what we keep as the only imx sdma firmware provider.

[1] http://lists.busybox.net/pipermail/buildroot/2021-January/300938.html

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Stephane Viau <stephane.viau@oss.nxp.com>
Cc: Gary Bisson <gary.bisson@boundarydevices.com>
[yann.morin.1998@free.fr: explain why we keep firmware-imx]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>