V4L/DVB (9239): Add support for rgb555 pixel formats to vivi

This patch adds RGB555 pixel format support to the vivi driver. Both
little endian and big endian versions are added. The driver follows
the RGB pixel format described in Table 2-2 of the V4L2 API spec,
_not_ the older BGR interpretation described in Table 2-1.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>