pinctrl: remove doc mention of the enable/disable API

This API has changed in commit 6e5e959d (pinctrl: API changes to support
multiple states per device).

Fixes: 6e5e959d ('pinctrl: API changes to support multiple states per device')
Cc: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>