mediatek: bananapi-r3: add overlay for ReSpeaker 2-mic hat

mediatek: bananapi-r3: add overlay for ReSpeaker 2-mic hat

Add DT overlay to allow using the ReSpeaker 2-mic hat connected to
the 26-pin header of the BananaPi BPi-R3.

Supported sample formats: S16_LE, S24_LE, S32_LE
Supported sample rates: 8/12/16/24/32/48/96/192 kHz

          |  BPi-R3  |  ReSpeaker 2-mic
  Signal  |  26-pin  |  40-pin
----------+----------+-------------------
 VCC 5V   |    2     |    2
 I2C SDA  |    3     |    3
 I2C SCL  |    5     |    5
 GND      |    6     |    6
 PCM CLK  |   12     |   12
 PCM FS   |   15     |   35
 PCM DIN  |   26     |   38
 GND      |   25     |   39
 PCM DOUT |   22     |   40

Once connected, install the driver and tell U-Boot to apply the DT
overlay:

opkg kmod-sound-soc-mt7986-wm8960
fw_setenv bootconf_base "config-mt7986a-bananapi-bpi-r3#mt7986a-bananapi-bpi-r3-respeaker-2mics"
reboot

To setup the ReSpeaker for Stereo out on the Headphone plug:

amixer sset 'Right Output Mixer PCM' on
amixer sset 'Left Output Mixer PCM' on
amixer sset 'PCM Playback -6dB' off
amixer sset 'O018 I150_Switch' on
amixer sset 'O019 I151_Switch' on
amixer sset 'O124 I032_Switch' on
amixer sset 'O125 I033_Switch' on
amixer sset 'Headphone' 121
amixer sset 'Playback' 255

Signed-off-by: Daniel Golle <daniel@makrotopia.org>