target-xtensa: add DBREAK data breakpoints

Add DBREAKA/DBREAKC SRs and implement DBREAK breakpoints as debug
watchpoints.

This implementation is not fully compliant to ISA: when a breakpoint is
set to an unmapped/inaccessible memory address it generates TLB/memory
protection exception instead of debug exception.

See ISA, 4.7.7.3, 4.7.7.6 for more details.

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