dm: pch: Add get_io_base op

On some newer chipset (eg: BayTrail), there is an IO base address
register on the PCH device which configures the base address of a
memory-mapped I/O controller.

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