mfd: intel-lpss: Save register context on suspend

All configurations are lost and the registers will have
default values when the hardware is suspended and resumed,
so saving the private register space context on suspend, and
restoring it on resume.

Fixes: 4b45efe8 (mfd: Add support for Intel Sunrisepoint LPSS devices)
Cc: stable@vger.kernel.org # v4.3+
Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>