target-arm: Implement ARMv8 MVFR registers

target-arm: Implement ARMv8 MVFR registers

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: Peter Maydell <[email protected]>
Reviewed-by: Peter Crosthwaite <[email protected]>