dm: pch: Add get_gpio_base op

x86 GPIO registers are accessed via I/O port whose base address is
configured in a PCI configuration register on the PCH device. Add
an op get_gpio_base to get the GPIO base address from PCH.

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