ASoC: wcd934x-dsp-cntl: Add misc device to control codec dsp

The codec DSP needs to be enabled only when there is use case that
utilizes the DSP. This way the codec DSP can be shutdown when it
is not used. Change adds misc device node that the user space can
use to trigger boot and shutdown of DSP.

CRs-Fixed: 1085213
Change-Id: Ie8bb9ed903e46b0914b4ba2630efa864c751c29b
Signed-off-by: Bhalchandra Gajare <gajare@codeaurora.org>