boards: mps2_an385: Enable SLIP networking

mps2_an385 is enabled for networking -> default networking is SLIP ->
SLIP selects UART_PIPE -> UART_PIPE requires UART_PIPE_ON_DEV defined
-> undefined leads to error on building some samples.

Fix all of these issues.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>