gfplatdetect: Add FnGBoot revision

The Flash-N-Go Boot bootloader propagates its version using ATAGS.
ATAGS, however, will be destroyed after bootup if an appended device tree
was found. Thus it needs to be saved to the device tree by the kernel
before.

Souch a kernel patch has been added and now get the FnGBoot info from
the device tree.

The Device Tree entry used for this is FnGBoot agnostic and was called
"atag-revision".