target-arm: Don't decode old cp15 WFI instructions on v7 cores

In v7 of the ARM architecture, WFI (wait for interrupt) is a first-class
instruction, but in previous versions this functionality was provided
via a cp15 coprocessor register. Add correct feature checks to the
decoding of the cp15 WFI instructions so that they behave correctly
for newer cores. In particular, the old 0,c7,c8,2 encoding used on
ARM940 has been reused for VA-to-PA translation in v6 and v7.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Adam Lackorzynski <adam@os.inf.tu-dresden.de>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
