overlays: Sets i2s_clk_producer as default for Hifiberry DACplusADC

As we have never released a (standard) DACplusADC board with onboard
clocks, we can simply use a fixed setup avoiding incompatibilities
with Pi5 during driver init. Setting 'hifiberry-dacplusadc,slave' in
the overlays disables the failing clock probing mechanism.

Removes 'slave' parameter description from README which is still
supported but not needed.

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