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/beacon/imx8mm/README b/board/beacon/imx8mm/README
index c65acef..32b24bc 100644
--- a/board/beacon/imx8mm/README
+++ b/board/beacon/imx8mm/README
@@ -11,7 +11,7 @@
 ======================================
 Note: $(srctree) is U-Boot source directory
 
-$ git clone https://source.codeaurora.org/external/imx/imx-atf
+$ git clone https://github.com/nxp-imx/imx-atf
 $ git lf-5.10.72-2.2.0
 $ make PLAT=imx8mm bl31 CROSS_COMPILE=aarch64-linux-gnu-
 $ cp build/imx8mm/release/bl31.bin $(srctree)