arch: xtensa: Add space for HiFi registers

Updates the xtensa_irq_base_save_area structure to include space
for saving/restoring the HiFi AudioEngine registers used by CP1.

The starting address of these HiFi AE registers also needs to be
referenced from assembly, so it is added to the set of symbols
symbols for which we need an offset to be auto-generated.

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