[ARM] arm64: Simplify and cleanup arch_vcpu_init()

We have 4 types of VCPUs supported on ARM64 which are Cortex-A8,
Cortex-A9, Cortex-A15, and Generiv-v8.

In future, more types of VCPUs (ARM32 & ARM64) will be added to
arch_vcpu_init() of ARM64 port hence this patch simplifies it
and makes it more readable.

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