Update PMIC and IMU interrupt handlers to be like FreeRTOS examples

This pull is more for comment & review rather than a firm pull request. I have no previous experience with FreeRTOS but the current code seems inconsistent with the examples of xEventGroupSetBitsFromISR() in event_groups.h and http://www.freertos.org/xEventGroupSetBitsFromISR.html

I have been wondering whether the PMIC is responsible for some lockups because the USB serial debug occasionally reports a flood of charge state changes. Perhaps there is noise on the MCP_STAT line or maybe the battery charger is oscillating between the charging and non-charging states as the dynamic CPU power usage effects the battery voltage.