xtensa: mmu: init: only clear enough entries in way 6

During MMU initialization, we clear TLB way 6 to remove all
identity mapping. Depending on CPU configuration, there are
certain number of entries per way. So use the number from
core-isa.h to clear enough entries instead of hard-coded
number 8. Specifying an entry number outside of permitted
range may result in CPU reacting in weird way so better to
avoid that.

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