ARM: at91/pm_slowclock: remove clocks which are already stopped when entering slow clock mode

Assume USB PLL and PLL B are already stopped before entering sleep mode.

Removed PLL B from slow clock code, all drivers are supposed to properly
unprepare clocks.

Signed-off-by: Sylvain Rochet <sylvain.rochet@finsecur.com>
Acked-by: Wenyou.Yang <wenyou.yang@atmel.com>
[nicolas.ferre@atmel.com: remove the warning printed in pm.c]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>