platform: nordic_nrf: Fix accidental delete of default SPIM3 IRQ handler

Fix accidental delete of the default SPIM3 IRQ handler.
Regression from: ec18ea271a47583d5f2257e046e1c49ace912570

Change-Id: I222cc5c7b7f641417282e4e7a6fe211bc7153d08
Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>
diff --git a/platform/ext/target/nordic_nrf/common/nrf5340/gcc/startup_nrf5340_s.S b/platform/ext/target/nordic_nrf/common/nrf5340/gcc/startup_nrf5340_s.S
index 30dd9fa..df7d579 100644
--- a/platform/ext/target/nordic_nrf/common/nrf5340/gcc/startup_nrf5340_s.S
+++ b/platform/ext/target/nordic_nrf/common/nrf5340/gcc/startup_nrf5340_s.S
@@ -300,6 +300,7 @@
     def_irq_handler      SERIAL1_IRQHandler
     def_irq_handler      SPIM4_IRQHandler
     def_irq_handler      SERIAL2_IRQHandler
+    def_irq_handler      SERIAL3_IRQHandler
     def_irq_handler      GPIOTE0_IRQHandler
     def_irq_handler      SAADC_IRQHandler
     def_irq_handler      TIMER0_IRQHandler