target-arm: Correct conversion of Thumb Neon dp encodings into ARM

We handle Thumb Neon data processing instructions by converting them
into the equivalent ARM encoding, as the two are very close. However
the ARM encoding should have bit 28 set, not clear. This wasn't causing
any problems because we don't actually look at that bit during decode;
however it is better to do the conversion correctly to avoid problems
later if we add checks to UNDEF on SBZ/SBO bits.

Signed-off-by: Juha Riihimäki <juha.riihimaki@nokia.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
