ARM: SoC cleanup on mach-at91 for 3.19

On Atmel AT91, the conversion to device tree is now considered complete,
and all machines that were not already converted in 3.18 are assumed to
be unused and dropped by the maintainer.

All remaining board files that were written in C are dropped, and the
ancient at91x40 sub-platform (based on an MMU-less ARM7) is removed
altogether.  Cleaning up the last pieces was great fun, so I took the
time to do some of the coding myself and removed several hundred code
lines that ended up unused after the board files were done.

There are still a couple of AT91 specific device drivers that are not
converted to DT (CF, USB-OTG) and currently not working, and the platform
itself is not "multiplatform"-enabled, but both issues are going to be
taken care of in the 3.20 cycle.

This is split out from the other cleanups purely based on the size
of the branch.
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