powerpc/cpuidle: Only clear LPCR decrementer wakeup bit on fast sleep entry

Currently when entering fastsleep we clear all LPCR PECE bits.

This patch changes it to only clear the decrementer bit (ie. PECE1), which is
the only bit we really need to clear here.  This is needed if we want to set
other wakeup causes like the PECEDH bit so we can use hypervisor doorbells on
powernv.  Also we no longer clear the MER bit as it should never be set in the
host anyway.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>