[POWERPC] PS3: Fix reading pm interval in logical performance monitor

ps3_read_pm (pm_interval) should return an actual HW register value
because the pm_interval register is a counter register.
This removes the shadow pm_interval register.

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