drivers: console: Refactor drivers to use shared init priority

Refactors all of the console drivers to use a shared driver class
initialization priority configuration, CONFIG_CONSOLE_INIT_PRIORITY, to
allow configuring console 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_DEFAULT to preserve
the existing default initialization priority for most drivers.

The driver-specific option, CONFIG_NATIVE_POSIX_CONSOLE_INIT_PRIORITY,
is left intact because the native posix console driver needs to
initialize after the UART console driver when both drivers are enabled.

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