x86: cpu: introduce scu_ipc_raw_command()

This interface will be used to configure properly some pins on
Merrifield that are shared with SCU.

scu_ipc_raw_command() writes SPTR and DPTR registers before sending
a command to SCU.

This code has been ported from Linux work done by Andy Shevchenko.

Signed-off-by: Georgii Staroselskii <georgii.staroselskii@emlid.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>