fix(test): compile error in test_irq_spurious_gicv2.c

Prior to this patch with PLATFORM_CORE_COUNT defined to 32 test_irq_spurious_gicv2.c fails to compile with the following error:
tftf/tests/runtime_services/trusted_os/tsp/test_irq_spurious_gicv2.c: In function ‘test_multicore_spurious_interrupt’:
tftf/tests/runtime_services/trusted_os/tsp/test_irq_spurious_gicv2.c:26:37: error: left shift count >= width of type [-Werror=shift-count-overflow]
   26 | #define CPU_TARGET_FIELD        ((1 << PLATFORM_CORE_COUNT) - 1)
      |                                     ^~
tftf/tests/runtime_services/trusted_os/tsp/test_irq_spurious_gicv2.c:232:48: note: in expansion of macro ‘CPU_TARGET_FIELD’
  232 |         gicv2_set_itargetsr_value(TEST_SPI_ID, CPU_TARGET_FIELD);
      |                                                ^~~~~~~~~~~~~~~~

Fix this by using 0xFF as the width of each field in GIC_ITARGETSR  is 8
bits.

Change-Id: I310cf7ada7230051ba3cedf6752f26c80528b198
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
diff --git a/tftf/tests/runtime_services/trusted_os/tsp/test_irq_spurious_gicv2.c b/tftf/tests/runtime_services/trusted_os/tsp/test_irq_spurious_gicv2.c
index eae5ccd..3f0c9cb 100644
--- a/tftf/tests/runtime_services/trusted_os/tsp/test_irq_spurious_gicv2.c
+++ b/tftf/tests/runtime_services/trusted_os/tsp/test_irq_spurious_gicv2.c
@@ -23,7 +23,7 @@
 #define TEST_SPURIOUS_ITERATIONS_COUNT	1000000
 
 #define TEST_SPI_ID		(MIN_SPI_ID + 2)
-#define CPU_TARGET_FIELD	((1 << PLATFORM_CORE_COUNT) - 1)
+#define CPU_TARGET_FIELD	0xFF
 
 static event_t cpu_ready[PLATFORM_CORE_COUNT];
 static volatile int requested_irq_received[PLATFORM_CORE_COUNT];