Re-enable flash clock test for NRF52 but with higher tolerance

The flash clock test is disabled for the NRF52 series. This change
re-enables the test but with a higher tolerance to accommodate the
high jitter on the current ticker implementation.

TESTS/mbed_hal/flash/functional_tests/main.cpp

@@ -28,7 +28,13 @@
 using namespace utest::v1;
 
 #define TEST_CYCLES         10000000
+
+#ifdef TARGET_NRF52
+/* The increased tolerance is to account for the imprecise timers on the NRF52. */
+#define ALLOWED_DRIFT_PPM   (1000000/50000)   //5.0%
+#else
 #define ALLOWED_DRIFT_PPM   (1000000/5000)    //0.5%
+#endif
 
 /*
     return values to be checked are documented at:
@@ -279,9 +285,7 @@ Case cases[] = {
     Case("Flash - erase sector", flash_erase_sector_test),
     Case("Flash - program page", flash_program_page_test),
     Case("Flash - buffer alignment test", flash_buffer_alignment_test),
-#ifndef MCU_NRF52
     Case("Flash - clock and cache test", flash_clock_and_cache_test),
-#endif
 };
 
 utest::v1::status_t greentea_test_setup(const size_t number_of_cases) {