target/xtensa: add internal/noop SRs and opcodes

Add two special registers: MMID and DDR:
- MMID is write-only and the only side effect of writing to it is output
  to the trace port, which is not emulated;
- DDR is only accessible in debug mode, which is not emulated.

Add two debug-mode-only opcodes:
- rfdd and rfdo do return from the debug mode, which is not emulated.

Add three internal opcodes for full MMU:
- hwwdtlba and hwwitlba are the internal opcodes that write a value into
  autoupdate DTLB or ITLB entry.
- ldpte is internal opcode that loads PTE entry that covers the most
  recent page fault address.
None of these three opcodes may appear in a valid instruction.

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