firmware-imx: update package description

Current firmware-imx version no longer contains ath6k and ar3k firmware.

Cc: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Acked-by: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>