drivers/serial: Add more USART ports for Silabs EFM32GG11B
drivers/serial: Add more USART ports for Silabs EFM32GG11B

Silicon Labs EFM32GG11B SoCs have more USART ports than
currently supported by the driver. Driver expanded with
support for USART4 and USART5.

Signed-off-by: Oane Kingma <2494258df89a84dd3197868146b44ec17d0dea76@interay.com>

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