PM: support for power-management API

[pthierry-ledger]

Add support for PM-related syscalls, so that a task with POWER capability is able to:

• set waiting external event/interrupt for awoking time
• request Sleep mode entering (starting with initial SLEEP mode that keep devices registers values set, STDBY to be considered later as it requires a signaling model for userspace drivers)
• support for auto-sleep at scheduling level, if only idle is eligible for a given amount of time (timeout to sleep support)