arch: xtensa: Carve space for saved HiFi regs

As the BSA can not be used when lazy HiFi context switching is
used, a more permanent and predictable location in which to store
the registers is required. To this end ...
  1. reserve some space in the arch-specific portion of the k_thread
     structure for those registers.
  2. clear that region when the thread is created.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>