[ARCH] Make DTS layout suitable for external DTB files

Currently, we only have built-in DTB compiled with Xvisor
binary but we also need to have scope of providing DTB file
externally via bootloader.

To achieve this, we must first have DTS layout similar to
Linux DTS layout so that bootloader such as u-boot and QEMU
can fixup DTB before passing it to Xvisor.

This patch does following changes to Xvisor DTS layout:
1. Rename /choosen to /chosen
2. Remove /host node and all child nodes of /host will
   be directly under / node
3. Use reg property in /memory node to inform Xvisor about
   available memory banks.

In simpler words, Xvisor DTS layout would be an extension of
Linux DTS layout. This means Xvisor DTS files will look very
similar to Linux DTS files except we will have /vmm, /guests,
and /templates nodes under / node which will not be present
in Linux DTS files.

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