ASRC task may collect timestamps on different tiles

[ed-xmos]

This makes the assumption that the system timer on both tiles is synch'ed. This seems to be the case:

Startup:
Tile: 0 timer: 84075821
Tile: 1 timer: 84075928
diff = 107
20ms later
Tile: 0 timer: 84275823
Tile: 1 timer: 84275930
diff = 107

But it may be better to grab the timestamp in the ISR of ASRC task which is on the same tile as ASRC + consumer. SPDIF does this in AN02003 but SPDIF/I2S modes do not.