HISI LPC: Stop using MFD APIs

The MFD APIs should only be used by drivers in
drivers/mfd. It is not worth splitting the driver to have
separate parts in drivers/bus and drivers/mfd, so just
drop MFD API usage.

As a solution, we will use the platform device APIs directly
to achieve the same as we had when using MFD APIs.

Signed-off-by: John Garry <john.garry@huawei.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>