serial: uart_nrfx_uarte: async compilation warning

Only compile in async related code when at least one UARTE instance has
enabled async mode. This fixes an "unused function" warning when
`CONFIG_UART_ASYNC_API` is enabled but no UARTE instances has async
enabled. This is possible when the async API is being used for an RTT
UART driver.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>