pinctrl: gemini: Support drive strength setting

The Gemini pin controller can set drive strength for a few
select groups of pins (not individually). Implement this
for GMAC0 and 1 (ethernet ports), IDE and PCI.

Cc: devicetree@vger.kernel.org
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>