samples: Bluetooth: hci_uart: Use lower IRQ priority SoC peripherals

Use lower IRQ proirities for SoC peripherals, example UART,
to reduce ISR latencies on the IRQs processed by the
Controller.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>

samples/bluetooth/hci_uart/dts/arm/nordic/override.dtsi

@@ -0,0 +1,7 @@
+/* Keep default IRQ priority low for peripherals to reduce Radio ISR latency.
+ * ARM Cortex-M4 lowest priority value of 5, i.e. considering Zephyr reserved 2
+ * levels for Exceptions and ZLI (if enabled).
+ * ARM Cortex-M0 lowest priority value of 3, i.e. we use it as Zephyr has no
+ * support for ZLI on Cortex-M0.
+ */
+#define NRF_DEFAULT_IRQ_PRIORITY 3