[ARM] arm32ve: Minor improvements to VFP save/restore

Current, VFP save/restore were directly checking HCPTR value programmed
in HW. This means arch_vcpu_switch() will have to restore HCPTR value
before doing VFP restore which can lead to bugs in-future. This patch
updates VCPU save/restore to check HCPTR value from private context
instead of HW.

Signed-off-by: Anup Patel <anup@brainfault.org>