tests: uart_async_api: Support nucleo_l4r5zi board

This commit adds the asynchronous UART API testing support on the
nucleo_l4r5zi board.

The usart3 module is used as the secondary loop-back UART, which is
required to run this test.

Signed-off-by: Shlomi Vaknin <shlomi.39sd@gmail.com>