fix: media: v4l2: abstract timeval handling in v4l2_buffer (v5.6)

See upstream commit :

  commit 77cdffcb0bfb87fe3645894335cb8cb94917e6ac
  Author: Arnd Bergmann <arnd@arndb.de>
  Date:   Mon Dec 16 15:15:00 2019 +0100

    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: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>