driver: serial: split silabs series 2 and series 0/1 boards usart driver

Split the USART driver into separate implementations for Silabs Series 2
and Series 0/1 boards. This change improves maintainability (especially
with the support of pin-ctrl and clock-ctrl on series 2 boards).

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