drivers/perf: Add Apple icestorm/firestorm CPU PMU driver

Add a new, weird and wonderful driver for the equally weird Apple
PMU HW. Although the PMU itself is functional, we don't know much
about the events yet, so this can be considered as yet another
random number generator...

Nonetheless, it can reliably count at least cycles and instructions
in the usually wonky big-little way. For anything else, it of course
supports raw event numbers.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>