ARM: perf: Preparatory work for Scorpion PMU support

Do some things to make the Krait PMU support code generic enough
to be used by the Scorpion PMU support code.

 * Rename the venum register functions to be venum instead of krait
   specific because the same registers exist on Scorpion

 * Add some macros to decode our Krait specific event encoding that's
   the same on Scorpion (modulo an extra region).

 * Drop 'krait' from krait_clear_pmresrn_group() so it can be used
   by Scorpion code

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>