cpuidle: mvebu: disable the bind/unbind attributes and use builtin_platform_driver

As the driver doesn't support unbinding, nor does it support arbitary
binding of devices, disable the bind/unbind attributes for this driver.
Also, as the driver has no remove function, it can never be modular,
so use builtin_platform_driver() to avoid the module exit boilerplate.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>