drivers: serial: gecko: Add device power management

Add basic device power management to USART driver. The initial
implementation only ensures that the TX FIFO is flushed before
allowing the system to go to sleep by polling the TXIDLE status
register.

This fixes an issue where the last 1-2 characters would be lost
if transmitted immediately before going to deep sleep.

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