Proper separation of RTM power up and power down responsabilities

[augustofg]

The responsibility of turning on / off the 12V payload power rail should land on the AMC card specific code instead of the RTM driver. The RTM specific code should have 'post power on' and 'pre power down' functions for initialisation / de-initialisation of devices powered by the payload power rail.

I suggest that the functions rtm_enable_payload_power and rtm_disable_payload_power should be migrated from the RTM driver to the AFC RTM functions, make them call rtm_enable_payload_power_post and rtm_disable_payload_power_pre that will be implemented in the RTM driver. Delete the rtm_quisce function to avoid confusion, the modules/rtm.c: RTM_Manage task should call rtm_disable_payload_power instead. Remember to do proper error propagation using the mmc_err type.