spi: ich: Don't require the PCH

spi: ich: Don't require the PCH

When booting from coreboot we may not have a PCH driver available. The
SPI driver can operate without the PCH but currently complains in this
case. Update it to continue to work normally. The only missing feature
is memory-mapping of SPI-flash contents, which is not essential.

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