[ARM] arm32: CPACR register is only available to ARMv6 or higher VCPUs

There was no CPACR register till ARMv6 hence ARMv5 or lower VCPUs
should get undefined instruction when accessing CPACR register.

This patch updates CP15 register read/write emulation as-per above.

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