target/arm/translate-a64: Don't underdecode system instructions

target/arm/translate-a64: Don't underdecode system instructions

The "system instructions" and "system register move" subcategories
of "branches, exception generating and system instructions" for A64
only apply if bits [23:22] are zero; other values are currently
unallocated. Correctly UNDEF these unallocated encodings.

Reported-by: Laurent Desnogues <[email protected]>
Signed-off-by: Peter Maydell <[email protected]>
Reviewed-by: Laurent Desnogues <[email protected]>
Message-id: 20190125182626[email protected]