ARM: Fix decoding of VFP forms of VCVT between float and int/fixed

ARM: Fix decoding of VFP forms of VCVT between float and int/fixed

Correct the decoding of source and destination registers
for the VFP forms of the VCVT instructions which convert
between floating point and integer or fixed-point.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Nathan Froyd <froydnj@codesourcery.com>