ASoC: wm_adsp: Add support for rev 2 firmware file format

Version 2 of the firmware file format includes length fields for the
various strings associated with control creation, to reduce file size.
However this does increase the parsing complexity slightly. This patch
adds support for the revision of the file format.

This patch also adds a new naming scheme for controls created from rev 2
firmware files. This version of the file format is commonly used to
add multiple controls per algorithm per memory region and the old
control naming scheme would cause multiple controls to have the same
name in this case.. Note that the naming scheme for older firmware
versions is left intact to ensure backwards compatibility.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>