Overlays: Add definitions for HiFiBerry 8 channel DAC8X

Dedicated overlay claiming all 4 data lanes of the designware
I2S0 module to drive 4x PCM5102. THe devices share BCLK and
LRCLK, therefore all outputs will always run at the same
samplerate and format.

Compatible only with PI5!

Signed-off-by: j-schambacher <joerg@hifiberry.com>