arm64: cpufeature: Add an overview comment for the cpufeature framework

arm64: cpufeature: Add an overview comment for the cpufeature framework

Now that Suzuki isn't within throwing distance, I thought I'd better add
a rough overview comment to cpufeature.c so that it doesn't take me days
to remember how it works next time.

Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/20200421142922.18950-9-will@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>