[POWERPC] pseries: phyp dump: Disable phyp-dump through boot-var

This adds a kernel command line option "phyp_dump", which takes a 0/1
value for disabling/ enabling phyp_dump at boot time.  Kdump can use
this on cmdline (phyp_dump=0) to disable phyp-dump during boot when
enabling itself.  This will ensure only one dumping mechanism is active
at any given time.

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