platform: nordic_nrf: Fix build issue for nRF54L15

Fix minor building issue when building upstream TF-M.

Change-Id: Ifa0b8ccfedf8f3cb2fa20713a339c0eafbd91cdd
Signed-off-by: Georgios Vasilakis <[email protected]>
(cherry picked from commit feaf99a)

Author: Vge0rge

platform/ext/target/nordic_nrf/common/nrf54l15/nrf54l15_init.c

@@ -9,6 +9,11 @@
 #include <hal/nrf_oscillators.h>
 #include <nrf_erratas.h>
 
+#ifndef BIT_MASK
+/* Use Zephyr BIT_MASK for unasigned integers */
+#define BIT_MASK(n) ((1UL << (n)) - 1UL)
+#endif
+
 /* This handler needs to be ported to the upstream TF-M project when Cracen is supported there.
  * The implementation of this is currently in sdk-nrf. We define it to avoid warnings when we build
  * the target_cfg.c file which is the same for both upsteam TF-M and sdk-nrf.