In order to be able to pass DT schema validation, change the GPU nodes'
unit-address to the standard notation. Previously this was using a "0,"
prefix that originated from a time when the top-level device tree node
contained #address-cells = <2>.

Note that this technically breaks backwards-compatibility with certain
older versions of the U-Boot bootloader because early versions used a
hard-coded DT path lookup to find the GPU node and perform some fixups
on it. However, this was changed to a compatible string based lookup in
April 2016, so it's reasonable to expect people to update U-Boot on the
systems that they want to use this updated kernel DTB with.
Signed-off-by: Thierry Reding <treding@nvidia.com>