[D18] add Bluetooth audio support

* enabled at u-boot: seco_config tool
* audio over SAI3 interface
  * SAI3 --> carrier board or onboard bluetooth (I2S2 SMARC | AUDIO BT)
* Filesystem support --> pulseaudio + bluez5
  * Yocto packages:
    pulseaudio-server pulseaudio-module-cli pulseaudio-misc
    pulseaudio-module-device-manager bluez5-noinst-tools bluez5-obex
    bluez5-testtools pulseaudio-module-bluetooth-discover
    pulseaudio-module-bluetooth-policy pulseaudio-module-bluez5-discover
    pulseaudio-module-bluez5-device pulseaudio-module-switch-on-connect
    pulseaudio-module-loopback
* Test details:
  root@seco-imx8mp-d18:~# pulseaudio -D
  root@seco-imx8mp-d18:~# bluetoothctl
  [bluetooth]# default-agent
  [bluetooth]# power on
  [bluetooth]# scan on
  ...
  Device 74:45:CE:40:2B:DD WH-CH510
  ...
  [bluetooth]# scan off
  [bluetooth]# trust 74:45:CE:40:2B:DD
  [bluetooth]# pair 74:45:CE:40:2B:DD
  [bluetooth]# connect 74:45:CE:40:2B:DD
  [WH-CH510]# info
  Device 74:45:CE:40:2B:DD (public)
    Name: WH-CH510
    Alias: WH-CH510
    Class: 0x00240404
    ...
  [WH-CH510]# exit
  root@seco-imx8mp-d18:~# pactl list sinks short | awk '{print $2}'
  root@seco-imx8mp-d18:~# paplay -d bluez_sink.74_45_CE_40_2B_DD.a2dp_sink test.wav