ASoC: img: Add documentation for SPDIF in controls

The Imagination Technologies SPDIF Input controller contains the following

controls:

name='IEC958 Capture Mask',index=0

This control returns a mask that shows which of the IEC958 status bits

can be read using the 'IEC958 Capture Default' control.

name='IEC958 Capture Default',index=0

This control returns the status bits contained within the SPDIF stream that

is being received. The 'IEC958 Capture Mask' shows which bits can be read

from this control.

name='SPDIF In Multi Frequency Acquire',index=0

name='SPDIF In Multi Frequency Acquire',index=1

name='SPDIF In Multi Frequency Acquire',index=2

name='SPDIF In Multi Frequency Acquire',index=3

This control is used to attempt acquisition of up to four different sample

rates. The active rate can be obtained by reading the 'SPDIF In Lock Frequency'

control.

When the value of this control is set to {0,0,0,0}, the rate given to hw_params

will determine the single rate the block will capture. Else, the rate given to

hw_params will be ignored, and the block will attempt capture for each of the

four sample rates set here.

If less than four rates are required, the same rate can be specified more than

once

name='SPDIF In Lock Frequency',index=0

This control returns the active capture rate, or 0 if a lock has not been

acquired

name='SPDIF In Lock TRK',index=0

This control is used to modify the locking/jitter rejection characteristics

of the block. Larger values increase the locking range, but reduce jitter

rejection.

name='SPDIF In Lock Acquire Threshold',index=0

This control is used to change the threshold at which a lock is acquired.

name='SPDIF In Lock Release Threshold',index=0

This control is used to change the threshold at which a lock is released.