V4L/DVB: add memory-to-memory device helper framework for videobuf

A mem-to-mem device is a device that uses memory buffers passed by
userspace applications for both their source and destination data. This
is different from existing drivers, which utilize memory buffers for either
input or output, but not both.

In terms of V4L2 such a device would be both of OUTPUT and CAPTURE type.

Examples of such devices would be: image 'resizers', 'rotators',
'colorspace converters', etc.

This patch adds a separate Kconfig sub-menu for mem-to-mem devices as well.

Signed-off-by: Pawel Osciak <p.osciak@samsung.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Reviewed-by: Vaibhav Hiremath <hvaibhav@ti.com>
Tested-by: Vaibhav Hiremath <hvaibhav@ti.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>