arch: xtensa: Add support for lazy HiFi ctx switching

When lazy HiFi context switching is enabled, the system starts with
the HiFi coprocessor disabled. Should the thread use that coprocessor,
it will generate an exception which in turn will enable the coprocessor
and save/restore the HiFi registers as appropriate. When switching
to a new thread, the HiFi coprocessor is again disabled.

For simplicity, there are no restrictions as to which thread is allowed
to use the coprocessor.

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