OMAP PM: Optimize cpufreq transition latency

Currently cpufreq transition latency value does not really reflect the actual
OMAP OPP transition delay. This patch has the actual latency value.
I did profile the DVFS latency on OMAP3430, OMAP3630 and OMAP4 for worstcase(MPU and Core together) and found that in none of these platforms, transiton value
goes beyong 10ms. Added a buffer of 20ms to avoid too frequent ondemand timer
expiry.
With this change, performance of ondemand governor is improved when tested
using cpufreqbench tool. Without this patch, cpufreq-bench reported ondemand
performance as 40% of performance governor, and with this patch it's around 70%
(using below procedure).

cpufreq-bench:
http://lwn.net/Articles/339862/
http://ftp.riken.go.jp/archives/Linux/suse/people/ckornacker/cpufreq-bench/

Command used for performance testing:
cpufreq-bench -l 50000 -s 100000 -x 50000 -y 100000 -g ondemand -r 5 -n 5 -v

Signed-off-by: Vishwanath BS <vishwanath.bs@ti.com>