drivers: serial: silabs_usart: Introduce pm_device_driver_init

Make use of pm_device_driver_init to perform driver initialization.
Implement PM suspend and resume, which performs the following actions:

* Enables/disables the USART
* Gates the USART clock
* Configures USART pins

Signed-off-by: Aksel Skauge Mellbye <aksel.mellbye@silabs.com>