fw/layer1: Add a priority field for sched_item

fw/layer1: Add a priority field for sched_item

Each item has a priority associated to it. The standard is :
-4    -> Responses processing
-3    -> L1S parameters changes
-2    -> [Reserved for TPU window setup]
-1    -> (anything)
 0..7 -> Commands relative to time slot n
         (relative to current l1s main timeslot)
 8    -> (anything)
 9    -> [Reserved for TPU window cleanup]
10    -> (anthing)

Note that with this modification, an item scheduled for the
current frame from within a call back won't have its priority
respected !

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>