[POWERPC] pseries: phyp dump: Inform kdump when phyp-dump is loaded

This adds /sys/kernel/phyp_dump_active so that kdump init scripts may
look for it and take appropriate action if this file is found.  This
file is only created when phyp_dump has been registered.

Signed-off-by: Manish Ahuja <mahuja@us.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>