media: v4l2: abstract timeval handling in v4l2_buffer

As a preparation for adding 64-bit time_t support in the uapi,
change the drivers to no longer care about the format of the
timestamp field in struct v4l2_buffer.

The v4l2_timeval_to_ns() function is no longer needed in the
kernel after this, but there is userspace code relying on
it to be part of the uapi header.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
[hverkuil-cisco@xs4all.nl: replace spaces by tabs]
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>