PM / devfreq: shut up kernel-doc warnings

There are 4 warnings there:
	drivers/devfreq/devfreq.c:707: warning: Function parameter or member 'val' not described in 'qos_min_notifier_call'
	drivers/devfreq/devfreq.c:707: warning: Function parameter or member 'ptr' not described in 'qos_min_notifier_call'
	drivers/devfreq/devfreq.c:717: warning: Function parameter or member 'val' not described in 'qos_max_notifier_call'
	drivers/devfreq/devfreq.c:717: warning: Function parameter or member 'ptr' not described in 'qos_max_notifier_call'

It turns that neither val nor ptr are actually used on those
function, so document as such.

Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>