ARM: dts: mt6589: Change compatible string for GIC

This patch changes the compatible string of the GIC to the
new "arm,cortex-a7-gic" which does reflect the actual hardware.

Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Olof Johansson <olof@lixom.net>