[ARM] Improve generic board to support ARMv5, ARMv6, ARMv7, and ARMv8 CPUs

Currently, the generic board support only supported ARMv8 CPUs. This patch
extends generic board to also support ARMv5, ARMv6, ARMv7 and ARMv7 (with
virtualization) CPUs.

With this patch in-place we can create multiplatform Xvisor binary will
boot on multiple boards for given ARM family.

The patch does not add DTS files for all ARM boards but eventually we will
have it for all ARM boards and board specific directories will go away.

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