spi: ich: Add mmio_base to struct ich_spi_platdata

It is useful to store the mmio base in platdata. It reduces the amount of
casting needed. Update the code and move the struct to the C file at the
same time, as we will need to use with of-platdata.

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