iPod Classic: capture support for CS42L55 codec

There are a couple of power saving options that can be selected using
defines, they configure the CODEC in a different way than OF does:

 MONO_MIC: jack microphone is connected to left channel, disabling
 right channel saves ~1 mW, there is no reason to not to do it.

 BYPASS_PGA: this option only applies to the line-in, OF does not
 bypass the PGA and configures it to 0 dB gain. At the beginning,
 this patch was written based on CODEC datasheet, bypassing PGA
 because it saves power and incrementes dinamic range ~1dB, i have
 used this setup for a while without problems. Finally this option
 was disabled at the last minute, i decided to do it after reviewing
 the OF and realizing that CS42L55 datasheet recommends to bypass the
 PGA only if the HW includes a couple of capacitors (see Typical
 Connection Diagram, Note 4), at this moment i don't know if Classic
 includes these capacitors (probably not). Anyway, i am not able to
 tell the difference listening to voice recodings.

TODO:
- Use variable PGA gain for jack microphone (it is fixed to +12 dB.
  as OF does).
- I am not a fan of having lots of unused #define options, these could
  be useful for a generic driver but actually this driver is Classic
  oriented, i am not sure if it could be considered disirable to
  eliminate them in the final version.

Change-Id: I3dadf2341f44d5e13f3847e6c9de4a76cd6f0918

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