drivers: uart_cmsdk_apb: Get clock frequency from DTS
drivers: uart_cmsdk_apb: Get clock frequency from DTS

The uart_cmsdk_apb driver used system clock frequency
as a base for baudrate calculation. This commit corrects
that by obtaining the needed value from DTS.

Signed-off-by: Piotr Zięcik <9ef3595b5f0f4fa8d0ceeb52cc0cf0411a0684dd@nordicsemi.no>

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