mfd: intel_soc_pmic_bxtwc: Convert to use new SCU IPC API

Convert the Intel Broxton Whiskey Cover PMIC driver to use the new SCU
IPC API. This allows us to get rid of the PMC IPC implementation which
is now covered in SCU IPC driver. We drop the error log if the IPC
command fails because intel_scu_ipc_dev_command() does that already.

Also move PMIC specific IPC message constants to the PMIC driver from
the intel_pmc_ipc.h header.

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>