iio: core: implement devm_iio_device_alloc/devm_iio_device_free

Add a resource managed devm_iio_device_alloc()/devm_iio_device_free()
to automatically clean up any allocations made by IIO drivers,
thus leading to simplified IIO drivers code.

In addition, this will allow IIO drivers to use other devm_*() API
(like devm_request_irq) and don't care about the race between
iio_device_free() and the release of resources by Device core
during driver removing.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Oleksandr Kravchenko <o.v.kravchenko@globallogic.com>
Tested-by: Oleksandr Kravchenko <o.v.kravchenko@globallogic.com>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>