imx: imx8: Update README with somewhat correct firmware versions

Current setup instructions for i.MX8QM MEK board include somewhat
conflicting instructions, so here's an attempt to fix it.

After trying both sets of firmwares, I can conclude that both of
them fail to work:

- no boot at all with imx-sc-firmware-0.7 / firmware-imx-7.6

- partial success with imx-sc-firmware-1.1 / firmware-imx-8.0:

U-Boot 2021.04-rc3-00135-ge0669a43c8 (Mar 08 2021 - 16:49:08 +0200)

CPU:   NXP i.MX8QM RevB A53 at 1200 MHz

Model: Freescale i.MX8QM MEK
Board: iMX8QM MEK
mu_hal_receivemsg timeout
Build: SCFW 65afe5f6, SECO-FW 65afe5f6, ATF a-20190
mu_hal_receivemsg timeout
sc_misc_get_boot_dev: res:16
Boot:  SD0
DRAM:  mu_hal_sendmsg timeout
sc_rm_is_memreg_owned: mr:0 res:21
...

Signed-off-by: Priit Laes <priit.laes@paf.com>