cpufreq: Add and use cpufreq_for_each_{valid_,}entry_idx()

cpufreq: Add and use cpufreq_for_each_{valid_,}entry_idx()

Pointer subtraction is slow and tedious. Therefore, replace all instances
where cpufreq_for_each_{valid_,}entry loops contained such substractions
with an iteration macro providing an index to the frequency_table entry.

Suggested-by: Al Viro <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Acked-by: Viresh Kumar <[email protected]>
Signed-off-by: Dominik Brodowski <[email protected]>
Signed-off-by: Rafael J. Wysocki <[email protected]>