powerpc/perf: macros for power9 format encoding

Patch to add macros and contants to support the power9 raw
event encoding format. Couple of functions added since some of the
bits fields like PMCxCOMB and THRESH_CMP has different width and location
within MMCR* in power9.

Signed-off-by: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>