target/arm: Allow users to set the number of VFP registers

Cortex A7 CPUs with an FPU implementing VFPv4 without NEON support
have 16 64-bit FPU registers and not 32 registers. Let users set the
number of VFP registers with a CPU property.

The primary use case of this property is for the Cortex A7 of the
Aspeed AST2600 SoC.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>

target/arm/cpu.h

@@ -924,6 +924,8 @@ struct ArchCPU {
     bool has_pmu;
     /* CPU has VFP */
     bool has_vfp;
+    /* CPU has 32 VFP registers */
+    bool has_vfp_d32;
     /* CPU has Neon */
     bool has_neon;
     /* CPU has M-profile DSP extension */