target/xtensa: Use the pre-defined MEMTXATTRS_UNSPECIFIED macro

Instead of using the hardcoded (MemTxAttrs){0} for no memory attributes
let's use the already defined MEMTXATTRS_UNSPECIFIED macro instead.

This is technically a change of behaviour as MEMTXATTRS_UNSPECIFIED sets
the unspecified field to 1, but it doesn't look like anything is
checking this field.

Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Acked-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
