For ARMv8 there are two changes to the MVFR media feature registers:
 * there is a new MVFR2 which is accessible from 32 bit code
 * 64 bit code accesses these via the usual sysreg instructions
   rather than with a floating-point specific instruction

Implement this.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NPeter Crosthwaite <peter.crosthwaite@xilinx.com>