devfreq: suppress platform driver bind / unbind feature

For simple-dev and devbw platform driver does not
support the manual bind / unbind feature through sysfs,
when the governor is registered and started.
Suppress the bind / unbind calls using driver attribute.

Change-Id: Ide843476bdc3870c48f6557097ee549e7a794a22
Signed-off-by: Santosh Mardi <gsantosh@codeaurora.org>