ACPI / LPSS: introduce a 'proxy' device to power on LPSS for DMA

The LPSS DMA controller does not have _PS0 and _PS3 methods. Moreover it can be
powered off automatically whenever the last LPSS device goes down. In case of
no power any access to the DMA controller will hang the system. The behaviour
is reproduced on some HP laptops based on Intel Bay Trail [1] as well as on
Asus T100 transformer.

This patch introduces a so called 'proxy' device that has the knobs to handle a
power of the LPSS island. When the system needs to program the DMA controller
it calls to the ACPI LPSS power domain callbacks that wake or suspend the
'proxy' device.

[1] http://www.spinics.net/lists/dmaengine/msg01514.html

Suggested-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: Scott Ashcroft <scott.ashcroft@talk21.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>