pinctrl: at91: update for drive strength options and tweaks

The drive strength patched introduced the atmel,sama5d-pinctrl
compatible string. Drive strength is now an option for the
CONFIG bits per pin. Also added note about MULTIDRIVE being
equivalent to open-drain output and added missing "s" at the
end of need everywhere in the bits descriptions.

Signed-off-by: Marek Roszko <mark.roszko@gmail.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>