intel_adsp: meteorlake: Initialize stack flush pointer SR

The simulator seems to drop garbage addresses (somewhere in the ROM it
looks like) into this SR at arbitrary times.  I don't know if this is
a hardware exception handler that we can't turn off, or a simulator
bug, or what.  But our code that assumes it will be cleared to zero or
valid is breaking.  Set it every time in every context switch for now
pending someone figuring out what's going wrong.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>
Signed-off-by: Anas Nashif <anas.nashif@intel.com>