imx: Suggest the NXP ATF github repo

As explained in the text at the bottom of the page
https://source.codeaurora.org/external/imx/imx-atf:

"QUIC repositories on this site will not receive any updates after
March 31, 2022, and will be deleted on March 31, 2023."

Point to the NXP ATF github repo instead.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Oliver Graute <oliver.graute@kococonnector.com>
Reviewed-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>
diff --git a/board/freescale/imx8qm_mek/README b/board/freescale/imx8qm_mek/README
index b1a4c6c..47fae3d 100644
--- a/board/freescale/imx8qm_mek/README
+++ b/board/freescale/imx8qm_mek/README
@@ -12,7 +12,7 @@
 Get and Build the ARM Trusted firmware
 ======================================
 
-$ git clone https://source.codeaurora.org/external/imx/imx-atf
+$ git clone https://github.com/nxp-imx/imx-atf
 $ cd imx-atf/
 $ git checkout origin/imx_4.14.78_1.0.0_ga -b imx_4.14.78_1.0.0_ga
 $ make PLAT=imx8qm bl31