[POWERPC] PS3: Remove unused os-area params

Updates for PS3 os-area startup params
o Remove some unused PS3 os-area startup params from struct saved_params.
o Rename ps3_os_area_init() to ps3_os_area_save_params().
o Zero mirrored header after saving params.

Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>