pinctrl: at91: add option to use drive strength bits

SAM9X60 uses high and low drive strengths. To implement this, in
at91_pinctrl_mux_ops::set_drivestrength and
at91_pinctrl_mux_ops::get_drivestrength we need bit numbers of
drive strengths (1 for low, 2 for high), thus change the code to
allow the usage of drive strength bit numbers.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Acked-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>