imx8mq_evk/README: add missing firmware extract step

Tested-by: NFabio Estevam <festevam@gmail.com>
Signed-off-by: NBaruch Siach <baruch@tkos.co.il>
Reviewed-by: NPeng Fan <peng.fan@nxp.com>

board/freescale/imx8mq_evk/README

@@ -18,6 +18,7 @@ Get the ddr and hdmi firmware
 Note: srctree is U-Boot source directory
 $ wget https://www.nxp.com/lgfiles/NMG/MAD/YOCTO/firmware-imx-7.9.bin
 $ chmod +x firmware-imx-7.9.bin
+$ ./firmware-imx-7.9.bin
 $ cp firmware-imx-7.9/firmware/hdmi/cadence/signed_hdmi_imx8m.bin $(srctree)
 $ cp firmware-imx-7.9/firmware-imx-7.9/firmware/ddr/synopsys/lpddr4*.bin $(srctee)