drivers: serial: silabs: introduce pm for silabs eusart

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

Also take PM locks to prevent deep sleep during TX and RX operations.

Signed-off-by: Martin Hoff <martin.hoff@silabs.com>