x86: ich6_gpio: Convert to use proper DM API

At present this GPIO driver still uses the legacy PCI API. Now that
we have proper PCH drivers we can use those to obtain the information
we need. While the device tree has nodes for the GPIO peripheral it is
not in the right place. It should be on the PCI bus as a sub-peripheral
of the PCH device.

Update the device tree files to show the GPIO controller within the PCH,
so that PCI access works as expected. This also adds '#address-cells'
and '#size-cells' to the PCH node.

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