xen/arm: Updates for extended regions support

xen/arm: Updates for extended regions support

This is a follow-up of
"b6fe410 xen/arm: Add handling of extended regions for Dom0"

Add various in-code comments, update Xen hypervisor device tree
bindings text, change the log level for some prints and clarify
format specifier, reuse dt_for_each_range() to avoid open-coding
in find_memory_holes().

Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
[stefano: fix typos]
Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>