ASoC: dt-bindings: qcom: sm8250: Document "pin-switches" and "widgets"

Some sound card setups might require extra pin switches to allow
turning off certain audio components. There are two real examples for
this in smartphones/tablets based on MSM8916:

  1. Analog speaker amplifiers connected to headphone outputs.

     The MSM8916 analog codec does not have a separate "Line Out" port
     so some devices have an analog speaker amplifier connected to one
     of the headphone outputs. A pin switch is necessary to allow
     playback on headphones without also activating the speaker.

  2. External speaker codec also used as earpiece.

     Some smartphones have two front-facing (stereo) speakers that can
     be also configured to act as an earpiece during voice calls. A pin
     switch is needed to allow disabling the second speaker during
     voice calls.

There are existing bindings that allow setting up such pin switches in
simple-card.yaml. Document the same for Qcom sound cards.

One variant of exa...