iio: core: pass parent device as parameter during allocation

The change passes the parent device to the iio_device_alloc() call. This
also updates the devm_iio_device_alloc() call to consider the device object
as the parent device by default.

Having it passed like this, should ensure that any IIO device object
already has a device object as parent, allowing for neater control, like
passing the 'indio_dev' object for other stuff [like buffers/triggers/etc],
and potentially creating iiom_xxx(indio_dev) functions.

With this patch, only the 'drivers/platform/x86/toshiba_acpi.c' needs an
update to pass the parent object as a parameter.

In the next patch all devm_iio_device_alloc() calls will be handled.

Acked-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>