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/doc/board/nxp/imx8mp_evk.rst b/doc/board/nxp/imx8mp_evk.rst
index b996ae0..e7cc7b3 100644
--- a/doc/board/nxp/imx8mp_evk.rst
+++ b/doc/board/nxp/imx8mp_evk.rst
@@ -16,7 +16,7 @@
 Get and Build the ARM Trusted firmware
 --------------------------------------
 
-Get ATF from: https://source.codeaurora.org/external/imx/imx-atf
+Get ATF from: https://github.com/nxp-imx/imx-atf
 branch: imx_5.4.70_2.3.0
 
 .. code-block:: bash