mfd: intel_soc_pmic: Add SCU IPC member to struct intel_soc_pmic

Both PMIC drivers (intel_soc_pmic_mrfld and intel_soc_pmic_bxtwc) will
be using this field going forward to access the SCU IPC instance.

While there add kernel-doc for the intel_soc_pmic structure.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>