ASoC: Add const to SOC_ENUM_*_DECL() macros

ASoC: Add const to SOC_ENUM_*_DECL() macros

Since these macros are supposed to be used for decalring const
objects, let's add the const modifier there.

The doubled const appearing in usages will be cleaned by later
patches.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Acked-by: Liam Girdwood <liam.r.girdwood@linux.intel.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>