xtensa: use highest available EPC/EPS regs in restore context

There may be Xtensa SoCs which don't have high enough interrupt
levels for EPC6/EPS6 to exist in _restore_context. So changes
these to those which should be available according to the ISA
config file.

Fixes #30126

Signed-off-by: Daniel Leung <daniel.leung@intel.com>