[ARM] arm32ve: Factor-out VFP switching and simplify CP15 switching

This patch primarily factors-out VFP switching into separate sources
cpu_vcpu_vfp.c and cpu_vcpu_vfp.h. We also introduce separate struct
arch_priv_vfp for maintaining VFP state.

The CP15 switching needed to be simplified so that we have clear
distinction between save and restore routines.

Signed-off-by: Sting Cheng <sting.cheng@gmail.com>
Signed-off-by: Anup Patel <anup@brainfault.org>