Add support for BERI statcounters.

BERI stands for Bluespec Extensible RISC Implementation, based on MIPS.

BERI has not implemented standard MIPS perfomance monitoring counters,
instead it provides statistical counters.

BERI statcounters have a several limitations:

• They can't be written
• They don't support start/stop operation
• None of hardware interrupt is provided on a counter overflow.

So make it separate to hwpmc_mips module and support process/system
counting mode only.

Sponsored by: DARPA, AFRL