Add PWM-DAC audio support for StarFive JH7110

Merge series from Hal Feng <hal.feng@starfivetech.com>:

This patchset adds PWM-DAC audio support for the StarFive JH7110 SoC.
The PWM-DAC module does not require a hardware codec, but a dummy codec is
needed for the driver. The dummy spdif codec driver, which is already
upstream, is compatible with the one which JH7110 PWM-DAC needed. So we
use it as the dummy codec driver for the JH7110 PWM-DAC module.