ASoC: sgtl5000: Improve VAG power and mute control

Backported from commit e6e98e2e05baa3862ac7d1d2334f88ec927c8f66.

VAG power control is improved to fit the manual [1]. This patch fixes as
minimum one bug: if customer muxes Headphone to Line-In right after boot,
the VAG power remains off that leads to poor sound quality from line-in.

I.e. after boot:
  - Connect sound source to Line-In jack;
  - Connect headphone to HP jack;
  - Run following commands:
  $ amixer set 'Headphone' 80%
  $ amixer set 'Headphone Mux' LINE_IN

Change VAG power on/off control according to the following algorithm:
  - turn VAG power ON on the 1st incoming event.
  - keep it ON if there is any active VAG consumer (ADC/DAC/HP/Line-In).
  - turn VAG power OFF when there is the latest consumer's pre-down event
    come.
  - always delay after VAG power OFF to avoid pop.
  - delay after VAG power ON if the initiative consumer is Line-In, this
    prevents pop during line-in muxing.

According to the data sheet [1], to avoid any pops/clicks,
the outputs should be muted during input/output
routing changes.

[1] https://www.nxp.com/docs/en/data-sheet/SGTL5000.pdf

Fixes: 9b34e6cc3bc2 ("ASoC: Add Freescale SGTL5000 codec support")
Signed-off-by: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>
Reviewed-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>

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