Device tree changes to boot FreeBSD on FVPs ...

FreeBSD does not understand #interrupt-map in a device tree. This prevents the
GIC from being set up correctly. This patch removes the #interrupt-map in the
device trees for the Base and Foundation FVPs. This enables correct boot of
FreeBSD on these platforms.

These changes have been tested with FreeBSD and an Ubuntu cloud image
(ubuntu-16.04-server-cloudimg-arm64-uefi1.img) to ensure compatibility with
Linux.

Change-Id: I1347acdcf994ec4b1dd843ba32af9951aa54db73
Signed-off-by: Achin Gupta <achin.gupta@arm.com>

AArch32: Update user-guide and add DTBs ...

This patch adds necessary updates for building and running Trusted
Firmware for AArch32 to user-guide.md. The instructions for running
on both `FVP_Base_AEMv8A-AEMv8A` in AArch32 mode and
`FVP_Base_Cortex-A32x4` models are added. The device tree files for
AArch32 Linux kernel are also added in the `fdts` folder.

Change-Id: I0023b6b03e05f32637cb5765fdeda8c8df2d0d3e