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/README.rockchip b/doc/README.rockchip
index 84caff8..5fcbbd1 100644
--- a/doc/README.rockchip
+++ b/doc/README.rockchip
@@ -73,7 +73,7 @@
 
    - Compile ATF
 
-     => git clone https://github.com/ARM-software/arm-trusted-firmware.git
+     => git clone https://github.com/TrustedFirmware-A/trusted-firmware-a.git
      => cd arm-trusted-firmware
 
      (export cross compiler path for Cortex-M0 MCU likely arm-none-eabi-)