pinctrl: at91-pio4: add support for drive strength

Add support for the drive strength configuration. Usually, this value is
expressed in mA. Since the numeric value depends on VDDIOP voltage, a
value we can't retrieve at runtime, the controller uses low, medium and
high to define the drive strength.

The PIO controller accepts two values for the low drive configuration: 0
and 1. Most of the time, we don't care about the drive strength. So we
keep the default value which is 0. The drive strength is advertised
through the sysfs only when it has been explicitly set in the device
tree i.e. if its value is different from 0.

Signed-off-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>