On Raspberry Pi, VideoCore firmware creates a device tree that contains
information about peripherals that were initialized by VideoCore based
on settings in config.txt. Normally this device tree is passed to the
Linux kernel via a pointer in the r2 register. A bootloader needs to
pass this device tree to the kernel, or some peripherals will not work
correctly.

Since the VideoCore device tree is not compatible with barebox, we can't
just pass it to barebox_arm_entry() as the internal barebox device tree.

This commit makes the prebootloader code copy the device tree from
VideoCore into a scrap RAM area just above the area reserved for the
bootloader. Board initialization code in the bootloader proper then
copies it into a file /vc.dtb. The bootloader environment is then free
to pass this file to the kernel at boot (e.g. via bootm -o).

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>