ASoC: rt5640: fix line out no sound issue

The power for line out was not turned on when line out is enabled.
So we add "LOUT amp" widget to turn on the power for line out.

Signed-off-by: John Lin <john.lin@realtek.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org