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 <peter.maydell@linaro.org>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>

target-arm/cpu.h

@@ -572,6 +572,7 @@ enum arm_cpu_mode {
 /* VFP system registers.  */
 #define ARM_VFP_FPSID   0
 #define ARM_VFP_FPSCR   1
+#define ARM_VFP_MVFR2   5
 #define ARM_VFP_MVFR1   6
 #define ARM_VFP_MVFR0   7
 #define ARM_VFP_FPEXC   8