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>
