drivers: serial: gecko: Use init macros
drivers: serial: gecko: Use init macros

Convert the present initialization code into initialization macros.
Since both the uart and the usart peripheral is implemented, two sets
of initialization macros are necessary.

Signed-off-by: Christian Taedcke <5dfb12f5b8690cb9c18e955016e7e3173b81d0d8@lemonbeat.com>

finikorg/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,Vudentz/zephyr,galak/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,galak/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,nashif/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,Vudentz/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr