pinctrl: add sleep state definition

There is an IDLE definition in the pinctrl framework, but for
ux500 SLEEP is more apropriate.

I've added some comments on the semantics of the common states
so as to avoid misunderstandings.

ChangeLog v1->v2:
- Fixed terminology "on"->"into".

Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>