ASoC: neo1973_wm8753: Move lm4857 specefic code to its own module

This patch moves the code for the lm4857 AMP from the neo1973_wm8753 sound
board driver to its own module.
The lm4857 is a generic AMP IC and not specific to the neo1973.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>