arch/arm/imxrt: support ramvectors in ITCM memory

This is done by initializing the ramvectors earlier in start so that
we can access the ITCM before the MPU gets configured

arch/arm/src/imxrt/imxrt_irq.c

@@ -436,7 +436,7 @@ void up_irqinitialize(void)
    * vector table that requires special initialization.
    */
 
-  arm_ramvec_initialize();
+  putreg32((uint32_t)g_ram_vectors, NVIC_VECTAB);
 #endif
 
   /* Set all interrupts (and exceptions) to the default priority */

arch/arm/src/imxrt/imxrt_start.c

@@ -37,6 +37,7 @@
 
 #include "arm_internal.h"
 #include "nvic.h"
+#include "ram_vectors.h"
 
 #include "imxrt_clockconfig.h"
 #include "imxrt_mpuinit.h"
@@ -216,6 +217,10 @@ void __start(void)
     }
 #endif
 
+#ifdef CONFIG_ARCH_RAMVECTORS
+  arm_ramvec_initialize();
+#endif
+
 #ifdef CONFIG_ARMV7M_STACKCHECK
   arm_stack_check_init();
 #endif