target-xtensa: define TLB_TEMPLATE for MMU-less cores

TLB_TEMPLATE macro specifies TLB geometry in the core configuration.
Make TLB_TEMPLATE available for region protection core variants,
defining 1 way ITLB and DTLB with 8 entries each.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
