cpufreq: helpers for walking the frequency table

Two new functions for getting the next higher and next lower frequencies
in the cpufreq table, based upon a frequency supplied in kHz.

This is useful for cpufreq governors that do not target frequencies
based upon a percentage or a pre-determined value, but instead access
the cpufreq table directly.

Signed-off-by: Mike Turquette <mturquette@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>