dt-bindings: pinctrl: add a 'drive-strength-microamp' property

This property allow drive-strength parameter in uA instead of mA.

Signed-off-by: Guillaume La Roque <glaroque@baylibre.com>
Acked-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>