Refactors all of the ADC drivers to use a shared driver class
initialization priority configuration, CONFIG_ADC_INIT_PRIORITY, to
allow configuring ADC drivers separately from other devices. This is
similar to other driver classes like I2C and SPI.

The default is set to CONFIG_KERNEL_INIT_PRIORITY_DEVICE to preserve the
existing default initialization priority for most drivers. The
exceptions are lmp90xxx, mcp320x, and mcux_adc16 drivers which have
dependencies on GPIO, SPI, and/or DMA drivers and must therefore
initialize later than the default device priority.

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