arm64: KVM: Switch the sys_reg search to be a binary search

Our 64bit sys_reg table is about 90 entries long (so far, and the
PMU support is likely to increase this). This means that on average,
it takes 45 comparaisons to find the right entry (and actually the
full 90 if we have to search the invariant table).

Not the most efficient thing. Specially when you think that this
table is already sorted. Switching to a binary search effectively
reduces the search to about 7 comparaisons. Slightly better!

As an added bonus, the comparison is done by comparing all the
fields at once, instead of one at a time.

Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>