target-xtensa: add basic checks to dcache opcodes

Check privilege level for privileged instructions (DHI, DHU, DII, DIU, DIWB,
DIWBI, DPFL are privileged), memory accessibility for instructions that
reference memory (all DH* and DPFL) and windowed register validity for all
data cache instructions.

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