ARM: at91: move platform-specific asm-offset.h to arch/arm/mach-at91

ARM: at91: move platform-specific asm-offset.h to arch/arm/mach-at91

<generated/at91_pm_data-offsets.h> is only generated and included by
arch/arm/mach-at91/, so it does not need to reside in the globally
visible include/generated/.

I renamed it to arch/arm/mach-at91/pm_data-offsets.h since the prefix
'at91_' is just redundant in mach-at91/.

My main motivation of this change is to avoid the race condition for
the parallel build (-j) when CONFIG_IKHEADERS is enabled.

When it is enabled, all the headers under include/ are archived into
kernel/kheaders_data.tar.xz and exposed in the sysfs.

In the parallel build, we have no idea in which order files are built.

 - If at91_pm_data-offsets.h is built before kheaders_data.tar.xz,
   the header will be included in the archive. Probably nobody will
   use it, but it is harmless except that it will increase the archive
   size needlessly.

 - If kheaders_data.tar.xz is built before at91_pm_data-offsets.h,
   the header will not be included in the archive. However, in the next
   build, the archive will be re-generated to include the newly-found
   at91_pm_data-offsets.h. This is not nice from the build system point
   of view.

 - If at91_pm_data-offsets.h and kheaders_data.tar.xz are built at the
   same time, the corrupted header might be included in the archive,
   which does not look nice either.

This commit fixes the race.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Link: https://lore.kernel.org/r/20190823024346.591-1-yamada.masahiro@socionext.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>