drivers: serial: silabs: Add new Kconfig symbol for async

Add a new Kconfig symbol (UART_SILABS_USART_ASYNC) to enable the async
API for the Silabs USART driver.
It is needed because the symbol (UART_ASYNC_API) is shared between uart
driver. In the case that you have multiple UART drivers enabled,
condition to enable async API for different uart driver may differ.

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