drivers: serial: Refactor drivers to use shared init priority Kconfig

Refactors all of the serial drivers to use a shared driver class
initialization priority configuration, CONFIG_SERIAL_INIT_PRIORITY, to
allow configuring serial 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 one
exception is uart_lpc11u6x.c which previously used
CONFIG_KERNEL_INIT_PRIORITY_OBJECTS.

This change was motivated by an issue on the frdm_k64f board where the
serial driver was incorrectly initialized before the clock control
driver.

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

drivers/serial/uart_esp32.c

@@ -447,7 +447,7 @@ DEVICE_DT_DEFINE(DT_NODELABEL(uart##idx),				       \
 		    &uart_esp32_data_##idx,				       \
 		    &uart_esp32_cfg_port_##idx,				       \
 		    PRE_KERNEL_1,					       \
-		    CONFIG_KERNEL_INIT_PRIORITY_DEVICE,			       \
+		    CONFIG_SERIAL_INIT_PRIORITY,			       \
 		    &uart_esp32_api);
 
 DT_INST_FOREACH_STATUS_OKAY(ESP32_UART_INIT);