aw_timer: rename driver from a10_timer

[mhorne]

Summary

This reflects its expanded support for platforms other than the A10.

Functions specific to A10/A13 retain the a10 prefix, but the majority of
names in the driver are changed, e.g. a10_timer_softc becomes
aw_timer_softc.

[manu]

Don't forget writing something in UPDATING for people who have a custom kernel config