[POWERPC] PS3: Add os-area rtc_diff set/get routines

Updates for PS3 os-area rtc_diff set/get routines
o Add a new routine ps3_os_area_set_rtc_diff().
o Rename ps3_os_area_rtc_diff() to ps3_os_area_get_rtc_diff().
o Remove static variable rtc_shift with calls to ps3_os_area_get_rtc_diff().

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