It's probably some custom ASIC marked with internal HP numbering system. Aside from dumping ROM or decapping it's impossible to tell what exactly is it. I don't think it is something like PA-8500, since
>The PA-8000 was used by:
>
> HP in its HP 9000 workstations and servers
> NEC in its TX7/P590 server
> Stratus Computer in its Continuum fault-tolerant servers
Using such microprocessors for embedded devices wouldn't be economical. It is probably some sort of RISC device but probably not PA-RISC.