pinctrl: clarify idle vs sleep states

This pure documentation fix tries to align the "idle" and
"sleep" pin states to the idle and suspend states from
runtime PM.

Cc: Patrice Chotard <patrice.chotard@st.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>