samples/subsys/zbus/uart_bridge: Add missing overlays for native_sim

During the hwmv1->v2 transition, overlays from a base
board were made to be shared with the variants.
So at that time all overlays for variants which were
just copies of the base overlay were removed.
After that in
zephyrproject-rtos/zephyr#71149
this shared/merged overlay behaviour was reverted,
but not all tests were fixed.

This is one of those. Let's fix it.

(cherry picked from commit 1c3edca)

Original-Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>
GitOrigin-RevId: 1c3edca
Change-Id: I560b2d50900c795cf7bea4593025604ba1616d61
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/zephyr/+/5465919
Tested-by: Jeremy Bettis <jbettis@chromium.org>
Commit-Queue: Jeremy Bettis <jbettis@chromium.org>
Reviewed-by: Jeremy Bettis <jbettis@chromium.org>
Tested-by: ChromeOS Prod (Robot) <chromeos-ci-prod@chromeos-bot.iam.gserviceaccount.com>

samples/subsys/zbus/uart_bridge/boards/native_posix_native_64.conf

@@ -0,0 +1 @@
+CONFIG_UART_NATIVE_POSIX_PORT_1_ENABLE=y

samples/subsys/zbus/uart_bridge/boards/native_sim_native_64.conf

@@ -0,0 +1 @@
+CONFIG_UART_NATIVE_POSIX_PORT_1_ENABLE=y