[BACKPORT] imxrt - interrupt serial storm, add DTCM and set up I and …

…D cache (#175)

* Serial Fixed interrupt storm

  The target would randomly hang in the serial isr.
  The priv->ie and the hardware were inconsistent.
  The isr used the priv->ie to gate offloading
  the RX data. Bang! Hung.

                  imxrt_disableuartint(priv, &ie);
                  ret = imxrt_setup(dev);

                  /* Restore the interrupt state */

                  imxrt_restoreuartint(priv, ie);
       interrupt->  Of no return
                  priv->ie = ie;

   On a fast cpu with FIFO, this will not work
   with out proper protections.

* Serial: Conditionally enable 9 bit mode

* armv7-mi/mpu.hi: Restructure API

   Preserve the existing API and enabled better granualriy on
   setting.

* Enable MPU for non protected builds to set cache

* mpuinit use symbolic values for addresses

* Allow DTCM on HEAP

* allocateheap Fix Coding style