powerpc/fadump: sysfs for fadump memory reservation

Add a sys interface to allow querying the memory reserved by FADump for
saving the crash dump.

Also added Documentation/ABI for the new sysfs file.

Signed-off-by: Sourabh Jain <sourabhjain@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191211160910.21656-7-sourabhjain@linux.ibm.com

powerpc/fadump: Reorganize /sys/kernel/fadump_* sysfs files

As the number of FADump sysfs files increases it is hard to manage all
of them inside /sys/kernel directory. It's better to have all the
FADump related sysfs files in a dedicated directory
/sys/kernel/fadump. But in order to maintain backward compatibility a
symlink has been added for every sysfs that has moved to new location.

As the FADump sysfs files are now part of a dedicated directory there
is no need to prefix their name with fadump_, hence sysfs file names
are also updated. For example fadump_enabled sysfs file is now
referred as enabled.

Also consolidate ABI documentation for all the FADump sysfs files in a
single file Documentation/ABI/testing/sysfs-kernel-fadump.

Signed-off-by: Sourabh Jain <sourabhjain@linux.ibm.com>
Tested-by: Michal Suchanek <msuchanek@suse.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191211160910.21656-4-sourabhjain@linux.ibm.com