ARM: dts: s5pv210: remove dedicated 'audio-subsystem' node

The 'audio-subsystem' node is an artificial creation, not representing
real hardware.  The hardware is described by its nodes - AUDSS clock
controller and I2S0.

Remove the 'audio-subsystem' node along with its undocumented compatible
to fix dtbs_check warnings like:

  audio-subsystem: $nodename:0: 'audio-subsystem' does not match '^([a-z][a-z0-9\\-]+-bus|bus|soc|axi|ahb|apb)(@[0-9a-f]+)?$'

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Tested-by: Jonathan Bakker <xc-racer2@live.ca>
Link: https://lore.kernel.org/r/20200907161141.31034-9-krzk@kernel.org