ARM: rpi: set fdt_high in the default environment

ARM: rpi: set fdt_high in the default environment

The ARM Linux kernel requires the DT to be in memory accessible early
during the boot process. This always happens naturally on the RPi 1,
since the maximum memory size of 512MiB, and additionally some of that
is reserved for use by the GPU. The RPi 2 has 1GiB of RAM (minus some
GPU usage), and so if the DT is relocated to the top of RAM, Linux cannot
access it. Prevent this from happening by setting fdt_high.

Cc: Daniel Stone <daniels@collabora.com>
Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>