mfd : Check if the other db8500 core is in WFI

This patch allows to check if the other core is in WFI
mode. It is the last check the idle routine has to do before
entering into the retention state.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>