pch: Add ioctl support

At present the PCH has 4 operations and these are reasonably widely used
in the drivers. But sometimes we want to add rarely used operations, and
each of these currently adds to the size of the PCH operations table.

Add an ioctl() method which can be easily expanded without any more impact
on the operations table.

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