[media] V4L: add two new ioctl()s for multi-size videobuffer management

A possibility to preallocate and initialise buffers of different sizes
in V4L2 is required for an efficient implementation of a snapshot
mode. This patch adds two new ioctl()s: VIDIOC_CREATE_BUFS and
VIDIOC_PREPARE_BUF and defines respective data structures.

Change-Id: Idb6c1e11f1665201f77c73168bb5a8de84b538eb
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
Signed-off-by: Ashray Kulkarni <ashrayk@codeaurora.org>