During system suspend, identify slaves which are configured
as wake sources and call pm_set_wakeup_source API for each of them.

Identifying if device may wake the system is done by checking if any
interrupt of that device is enabled in GICD_ISENABLER when the APU is
about to enter SUSPEND_TO_RAM state. If such interrupt is found,
pm_set_wakeup_source is called with corresponding PM node ID as
argument.

Signed-off-by: Filip Drazic <filip.drazic@aggios.com>