powerpc/powermac: Use str_enabled_disabled() and str_on_off() helpers

Remove hard-coded strings by using the str_enabled_disabled() and
str_on_off() helper functions.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250117114625.64903-2-thorsten.blum@linux.dev