Add basic support for the Krait CPU PMU. This allows us to use
the architected functionality of the PMU.

This is based on code originally written by Ashwin Chaugule and
Neil Leeder [1].

[1] https://www.codeaurora.org/cgit/quic/la/kernel/msm/tree/arch/arm/kernel/perf_event_msm_krait.c?h=msm-3.4

Cc: Neil Leeder <nleeder@codeaurora.org>
Cc: Ashwin Chaugule <ashwinc@codeaurora.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>