spi: ich: Use compatible strings to distinguish controller version

At present ich spi driver gets the controller version information via
pch, but this can be simply retrieved via spi node's compatible string.

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