cpufreq: make the "scaling_cur_freq" sysfs entry pollable

Wakeup userspace poll on change of cpu frequency. The userspace
may then take action to change the power/performance
characteristics of the device.

Change-Id: I3030b22084fe7e0143b978a198ddcc579e7d6e83
Signed-off-by: Amar Singhal <asinghal@codeaurora.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>