PM / devfreq: make devfreq-event explicitly non-modular

The Kconfig currently controlling compilation of this code is:

menuconfig PM_DEVFREQ_EVENT
	bool "DEVFREQ-Event device Support"

...meaning that it currently is not being built as a module by anyone.

Lets remove the modular code that is essentially orphaned, so that
when reading the driver there is no doubt it is builtin-only.

This code wasn't using module_init, so we don't need to be concerned
with altering the initcall level here.

We don't replace module.h with init.h since the file already has that.
But we do add export.h since this file does export some symbols.

We also delete the MODULE_LICENSE tag etc. since all that information
is already contained at the top of the file in the comments.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Acked-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>