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