samples: logging: logger: Add configuration for nrf52840dk

Added board specific configuration which enables use of
asynchronous UART API in UART log backend.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>

samples/subsys/logging/logger/boards/nrf52840dk_nrf52840.conf

diff --git a/samples/subsys/logging/logger/boards/nrf52840dk_nrf52840.conf b/samples/subsys/logging/logger/boards/nrf52840dk_nrf52840.conf
new file mode 100644
index 0000000..f6bef63
--- /dev/null
+++ b/samples/subsys/logging/logger/boards/nrf52840dk_nrf52840.conf
@@ -0,0 +1,3 @@
+CONFIG_UART_ASYNC_API=y
+CONFIG_UART_0_ASYNC=y
+CONFIG_LOG_BACKEND_UART_ASYNC=y