arm: mach-k3: am625: Provide a way to obtain boot device for non SPLs

Introduce get_boot_device() to obtain the booting device. Make it also
available for non SPL builds so u-boot can also know the device it is
booting from.

Signed-off-by: Wadim Egorov <w.egorov@phytec.de>
diff --git a/arch/arm/mach-k3/include/mach/hardware.h b/arch/arm/mach-k3/include/mach/hardware.h
index af982e7..c724450 100644
--- a/arch/arm/mach-k3/include/mach/hardware.h
+++ b/arch/arm/mach-k3/include/mach/hardware.h
@@ -107,4 +107,5 @@
 	u32 num_components;
 };
 
+u32 get_boot_device(void);
 #endif /* _ASM_ARCH_HARDWARE_H_ */