cobalt/arm64: prepare-kernel: Map aarch64 to arm64

Various build systems and also the kernel uses aarch64 to refer to
arm64, map aarch64 and arm64 to the same thing.
Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

scripts/prepare-kernel.sh

@@ -278,7 +278,7 @@ while : ; do
    arm)
       linux_arch=arm
       ;;
-   arm64)
+   arm64|aarch64)
       linux_arch=arm64
       ;;
    *)