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 2f9095e2bf59 ("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>
diff --git a/arch/arm/mach-k3/Makefile b/arch/arm/mach-k3/Makefile
index 1bd5233..3101f57 100644
--- a/arch/arm/mach-k3/Makefile
+++ b/arch/arm/mach-k3/Makefile
@@ -26,3 +26,4 @@
 endif
 obj-y += common.o security.o
 obj-$(CONFIG_SOC_K3_AM625) += am62x/
+obj-$(CONFIG_SOC_K3_AM642) += am64x/