ENGR00230920-1 HDMI AUDIO: add support for HDMI audio module build

ENGR00230920-1 HDMI AUDIO: add support for HDMI audio module build

HDMI driver can be divided into 3 parts: machine driver, platform
driver, codec driver.To support HDMI AUDIO loadable module build, HDMI
machine driver should be built as loadable.

In this patch, adjust HDMI audio driver's struct: move HDMI audio platform
 driver to snd-soc-imx-objs(snd-soc-imx-objs would always be
build-in).In this case, user need only build HDMI AUDIO machine driver
as loadable.

Signed-off-by: Chen Liangjun <b36089@freescale.com>