[BEETLE] Disable fcache stats if uvisor is defined

This patch disables the fcache stats into mbed_sdk_init if uvisor is
defined in order to prevent MEMMANAGEMENT faults during boot.

Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>

hal/targets/hal/TARGET_ARM_SSG/TARGET_BEETLE/mbed_sdk_init.c

@@ -23,8 +23,10 @@ void mbed_sdk_init(void) {
     EFlash_DriverInitialize();
     EFlash_ClockConfig();
 
+#if !defined(FEATURE_UVISOR) || !defined(TARGET_UVISOR_SUPPORTED)
     /* Enable Flash Cache Stats */
     FCache_DriverInitialize();
     FCache_Enable(1);
     FCache_Invalidate();
+#endif
 }