Update the ARM trusted firmware git URL

The primary upstream URL for tf-a has moved, in some cases
things like tags are not always pushed to the old URL so
update the URLs to the primary upstream project URL.

Signed-off-by: Peter Robinson <pbrobinson@gmail.com>
diff --git a/doc/board/rockchip/rockchip.rst b/doc/board/rockchip/rockchip.rst
index 0f9cb40..a275e96 100644
--- a/doc/board/rockchip/rockchip.rst
+++ b/doc/board/rockchip/rockchip.rst
@@ -166,7 +166,7 @@
 
 .. code-block:: bash
 
-        git clone --depth 1 https://github.com/ARM-software/arm-trusted-firmware.git
+        git clone --depth 1 https://github.com/TrustedFirmware-A/trusted-firmware-a.git
         cd arm-trusted-firmware
         make realclean
         make CROSS_COMPILE=aarch64-linux-gnu- PLAT=rk3399