dove: fixed bug in the PMU interrupts handler when clearing the cause register

BugLink: http://bugs.launchpad.net/bugs/611634

this patch clears the set bits in the PMU interrupt cause register by writing
zero to the bit that caused the interrupt and also to the other bits that are not
set.

Signed-off-by: Eric Miao <eric.miao@canonical.com>
Signed-off-by: Brad Figg <brad.figg@canonical.com>