x86: broadwell: Add a pinctrl driver

GPIO pins need to be set up on start-up. Add a driver to provide this,
configured from the device tree.

The binding is slightly different from the existing ICH6 binding, since that
is quite verbose. The new binding should be just as extensible.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Bin Meng <bmeng.cn@gmail.com>