ASoC: simple-amplifier: remame dio2125 documentation

The dio2125 is simple enough that we can make it a generic component.
Rename the the dio2125 documentation to simple-amplifier to
prepare this change.

Suggested-by: Nicolò Veronese <nicveronese@gmail.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Mark Brown <broonie@kernel.org>