arm: mach-k3: am642: Provide a way to obtain boot device for non SPLs

Let's provide get_boot_device() for AM64, similar to what we did
in commit 2f9095e2 ("arm: mach-k3: am625: Provide a way to obtain boot
device for non SPLs") for AM62.

Signed-off-by: Wadim Egorov <w.egorov@phytec.de>