tests/drivers uart_mix_fifo_poll: Speed up for nrf52_bsim

Increase the UART speed to 1Mbps for the simulated nrf52
to reduce the amount of time spent busy waiting, and
therefore the real time duration of the test.

Signed-off-by: Alberto Escolar Piedras <[email protected]>

Author: aescolar

tests/drivers/uart/uart_mix_fifo_poll/boards/nrf52_bsim.overlay

@@ -1,3 +1,8 @@
 /* SPDX-License-Identifier: Apache-2.0 */
 
 #include "nrf52840dk_nrf52840.overlay"
+
+
+dut: &uart0 {
+	current-speed = <1000000>;
+};