topology: An element can refer to multipe data sections in text conf file

topology: An element can refer to multipe data sections in text conf file

Previously in text conf file, an element can only refer to one data
section (SectionData). Now it can also refer to a list of data sections.
Thus users can split groups of firmware parameters to multiple data
sections, and put them all in the reference list.

Finally, data of these data sections will be merged, in the same order as
they are in the reference list, as the element's private data for kernel.

We still support the original syntax of reference to a single data
section. The doc is updated for the syntax extension.

Signed-off-by: Mengdong Lin <mengdong.lin@linux.intel.com>
Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>