cpufreq: Make the "scaling_governor" sysfs node pollable

A userspace module programs different qos-rules depending on the
governor running in the system. Make the governor node
pollable, so that the userspace module can be triggered when
the value of the governor changes.

change-Id: Ic89c77c7d16b0f8954d59a211612e9a8e98a2c28
Signed-off-by: Amar Singhal <asinghal@codeaurora.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>