drivers: i2s: Refactor drivers to use shared init priority

Refactors the remaining I2S drivers to use the shared driver class
initialization priority configuration, CONFIG_I2S_INIT_PRIORITY, to
allow configuring I2S drivers separately from other devices. This is
similar to other driver classes.

The default is set to CONFIG_KERNEL_INIT_PRIORITY_DEVICE to be
consistent with other driver classes.

Signed-off-by: Maureen Helm <maureen.helm@intel.com>