dbcool(4) support for ADT7460 on i2c

[Taylor R Campbell <campbell%mumble.net@localhost>]

My aluminium PowerBook G4 12" has a fan device that Open Firmware
describes as `adt7460' in the `compatible' property.  I looked at the
data sheet, and it seemed pretty similar to the ADT7463 rev 0x27 (in
fact, in device and revision ids, it is indistinguishable).  I tried
changing sys/arch/macppc/dev/dbcool_ki2c.c so that it would match
devices identified as this, and it seems to work perfectly for me.
Ought I to submit a PR, or can someone just make (or test and make)
this change?

(Unfortunately, this didn't help me to figure out why, after
disassembling and reassembling the PowerBook to clean it out, the fan
is blowing at ten thousand rounds per minute, as envstat(8) now
gleefully reports.)